School Science Lessons
UNPhysics1a Physics experiments
Colour, gas discharge tubes, light sources, reflection, refraction, sound, waves
Please send comments to:
2014-11-17 sp
Table of contents
4.100 Colour
4.500 Gravity
4.200 Light sources, producing light
4.300 Reflection of light at flat surfaces, plane mirrors
4.400 Refraction of light at curved surfaces, magnifiers
4.91 Refraction of waves in a ripple tank
4.101 Ear and hearing
4.102 Voice and speaking

4.100 Colour
4.117 Absorption spectrum of sodium
4.138 Colours of oil films
4.140 Colours of opaque objects
4.137 Colours of soap films
4.132 Colours of sunlight, rainbow
4.144 Colours of the blue sky and the sunset
4.145.1 Colours of water
4.145 Colours of the sea
4.139 Colours of transparent objects, colour filters
4.114 Dispersion, spectrum with a ray box
4.133 Electromagnetic radiation
4.115 Emission spectrum
4.116 Incandescent lamp
4.135 Infrared rays source
4.143 Mix coloured lights
4.141 Mix coloured pigments, blue and yellow chalk
4.142 Rotate colour discs
4.134 Spectroscope, diffraction grating
4.136 Ultraviolet light source

4.500 Gravity
4.148 Acceleration of marbles down an incline
4.146 Balance with a metre stick, stationary meeting point, centre of mass, centre of gravity
4.147 Ball bearings fall together
4.150 Coupled pendulums
4.154 Falling washers on a string
4.153 Three-holes can, 3-hole can, a vase with three holes, spouting cylinder, Mariotte's flask

4.200 Light sources, producing light
4.117 Absorption spectrum of sodium
4.103.1 Candoluminescence
4.115 Emission spectrum
4.116 Incandescent lamp
4.135 Infrared rays source
4.120 Light rays through lenses
4.105 Light travels in straight lines, pinhole magnifier
4.102 Low voltage light source
4.104 Luminance and illuminance, candela, candlepower, lumen, lux
4.103 Luminescence

4.300 Reflection of light at flat surfaces, plane mirrors
4.111 Laws of reflection using a ray box
4.109 Mirror images, (inversion, lateral inversion)
4.110 Ray box for beams of light
4.112 Reflection from a concave mirror with a ray box
4.113 Reflection from a convex surface
4.106 Reflection of beams of light
4.108 Reflection with a smoke box
4.107 Smoke box to study light rays

4.400 Refraction of light at curved surfaces, magnifiers
4.127 Critical angle and total internal reflection
4.120.1 Focal length of a convex lens
4.128 Image with a convex lens, magnifying glass
4.120 Light rays through lenses
4.129.1 Magnifiers, magnifying glass
4.129 Magnifying power of a lens
4.125 Measure refractive index
4.131 Optical bench to study lenses
4.126 Refraction from air to water
4.121 Refraction in a smoke box
4.122 Refraction in water illusions, pool depth, bent stick, rising coin
4.123 Refractive index using real depth and apparent depth
4.124 Refractive index using real depth and apparent depth, air to liquid
4.130 Water drop magnifier, water lens

4.67 Simple compass needle
See diagram 4.67.1: Simple compass needles 1. See diagram 4.67.2: Simple compass needles 2.
1. Magnetize a sewing needle by stroking it with a bar magnet. Make a simple compass by:
1.1 pushing the magnetized needle through cardboard and suspending it on a thread,
1.2 pushing the needle through the projections of a cloth-covered button,
1.3 attaching the needle to a strip of cardboard and balancing it over an inverted test-tube supported on a long pin.
Label the end of the magnet that tends to point north.
2. Make another simple compass needle by pushing two magnetized sewing needles through the holes of a large press stud and balancing it on the end of a needle pushed into a cork. Push a magnetized needle through thin cardboard and suspend it on a thread inside a glass jar.
3. Compare the north direction shown by a plotting compass with the directions shown by the simple compass needles. A compass needle is marked "N" at on end. This end points towards the north magnetic pole so it is the "north-seeking pole" of the magnet. The other end is the "south-seeking pole".

4.68 Magnetic dip
See diagram 4.68: Measuring magnetic dip angles
Push a steel knitting needle through cylindrical cork at right angles to its long axis. Push a pin into the centre of each end of the cork to act as an axle. Balance the cork through its axle of pins on knife edges. Magnetize the steel knitting needle using a magnetizing coil, see 2.166. Balance the cork again. The earth's magnetic field pulls one end of the needle downwards. Fix a spirit level, or a glass tube containing a bubble in water, above the knitting needle. Use a protractor to measure the angle of dip between the horizontal spirit level and the knitting needle. At the north magnetic pole or at the south magnetic pole the needle should point straight down. At the equator the knitting needle will be about parallel to the spirit level.

4.69 Make a magnetizing coil
See diagram 4.69: A magnetizing coil
Use glass tubing wound with close turns of insulated copper wire to magnetize steel knitting needles.

4.70 Freely-suspended magnet
See diagram 4.70: Suspended magnet
Use loops of cotton to suspend two magnets freely. Bring each pole of the two magnets close to, but not touching, each other. Show that like poles repel and unlike poles attract.

4.71 Natural magnets
A form of magnetite, iron (II, III) oxide, called lodestone, acts as a magnet when freely suspended. It was probably first discovered in China where they used it for the first magnetic compasses.

4.72 Artificial magnets
Look for low-cost artificial magnets in discarded loudspeakers, telephone receivers and other equipment. Artificial magnets have different shapes, e.g. "Alnico", horseshoe magnet, pairs of bar magnets with a soft iron keeper, cylindrical magnets. Store artificial magnets in pairs in a box, north to south, and south to north. Be careful! Keep magnets away from computer diskettes (floppy discs) and colour television screens.

4.73 Identify magnetic substances
Collect objects made of different substances, e.g. paper, wax, brass, zinc, iron, steel, glass, cork, rubber, aluminium, copper, gold, silver, wood, tin. Test each object with a magnet to see which objects a magnet attracts or does not attract. Bring a soft iron wire and hard steel or piano wire near a compass needle to see if a magnetic field affects it.

4.74 Magnetic poles and pin chains
1. Use a 6 cm length of iron wire. Draw one end of a magnet along it once only and in one direction from end to end. Lay the wire on a piece of paper then test for magnetism by sprinkling iron filings over it. The iron filings are not attracted equally along its whole length. The areas of strongest attraction are the magnetic poles of the piece of wire. Use adhesive tape to removes iron filings from a strong magnet.
2. Pick up a pile of pins with the magnet. Leave one pin attached to the magnet. Take off another pin and bring it close the end of the first pin. They will stick together by magnetic force. Connect all the pins to make a magnetic pin chain.

4.75 Cut an iron wire magnet
Cut in half a magnetized steel wire. Test both ends of each broken portion. The magnetism found on each side of the break has opposite polarity. Cut off a very small piece of the wire magnet and test it with iron filings. The smallest piece of the wire is a magnet with opposite poles.

4.76 Magnetic fields in two dimensions
See diagram 4.76.1: Magnetic fields 1 | See diagram 4.76.2: Magnetic fields 2: 4.76.2 A Neutral point, B Soft iron, C Induced S pole, D induced N pole
1. Sprinkle iron filings evenly on a thin card. Hold the card high over a bar magnet then carefully lower it until it almost touches the magnet. Tap the card gently with the end of a pencil. The iron filings move into a pattern showing the magnetic field.
2. Repeat the experiment with two bar magnets in different positions. The iron filings tend to line up in "lines of force", "field lines". Hold a plotting compass above the lines of force and compare their direction with the direction of the compass needle. Put an unmagnetized piece of soft iron near two bar magnets on the desk and observe the interesting magnetic fields formed.
3. Make permanent records of the magnetic field as follows:
3.1 Spray over the iron filings with a paint sprayer.
3.2 Replace the card with photographic paper in a dark room. Shine a bright light on it and develop the print.
3.3 Dip a white sheet of paper in melted wax. Let it cool then sprinkle iron filings on the solid wax. Hold the paper over a strong magnet to allow the iron filings to move into lines of force patterns. Hold a hot iron over the iron filings to let them sink into the wax.
3.4 You can photocopy iron filings on transparent paper but do not use a strong magnet near a photocopy machine.

4.77 Magnetic fields in three dimensions
Add oil to iron filings in a container. Shake to see if the filings will go into suspension in the oil. Use a concentration of oil that allows the iron filings to remain suspended then bring a magnet to the container to develop a pattern of iron filings in three dimensions. Make a permanent record using water glass or liquid plastic.

4.79 Horseshoe electromagnets
See diagram 4.79: Horseshoe electromagnets
Do NOT use a lead cell accumulator, car battery, for this experiment because the resistance of these coils is low and the current will be too large with a significant fire risk. If you use horseshoe magnets or C-shape magnets, wind the coil in opposite directions on each arm of the magnet. Use a U-shape piece of iron. Wind a coil of three layers of bell wire on each straight arm of the iron, but not on the curving part. Leave 30 cm of wire before you start winding the coil from the end of one arm. Cross to the other arm. Wind a coil of three layers and leave 30 cm of wire at the end. Wind three layers of wire on this pole then wind insulating tape around the wires so they cannot unwind. Remove the insulation from the ends of the coil, connect the horseshoe magnet in series with a car headlight bulb, connect to two dry cells or lead cell accumulators, and test the poles of the electromagnet. One pole should be a north pole and the other pole should be a south pole. If each pole has the same polarity, you have wound the second coil in the wrong direction so you must unwind the coil and rewind it in the opposite direction. Use the magnet to attract different things. Compare the strength of this electromagnet with the cylindrical electromagnet.

4.80 Test the strength of electromagnets
Do not use lead cell accumulators for this experiment because the resistance of these coils is low and the current will be large with a significant fire risk.
1. Wind 25 turns of bell wire on a straight iron bolt and connect one dry cell or lead cell accumulator to the ends of the wire. Record the number of pins or paper clips you can pick up with the electromagnet.
2. Repeat the experiment with two dry cells or lead cell accumulators connected in series.
3. Wind on 25 more turns of wire in the same direction. Join them to the first 25 turns. Repeat the experiment.
4. Repeat the experiment with two dry cells or lead cell accumulators connected in series.
5. Wind on another 50 turns. Join them to the first 50 turns. Repeat the experiment.
6. Repeat the experiment with two dry cells or lead cell accumulators connected in series. Remove 50 turns and rewind them on the bolt in the opposite direction.
7. With 100 turns so wound, repeat the experiment with two dry cells or lead cell accumulators connected in series.

4.81 Magnetic field from electric current in a wire
Pull 25 cm of insulated copper wire through a hole in the centre of a small white piece card. Connect the ends of the wire to a battery through a car headlight bulb. Fix the card in a horizontal position. Fix the wire in a vertical position. Sprinkle iron filings evenly on the card. Switch on the current. Tap the card gently with the end of a pencil. The iron filings move into a pattern showing the magnetic field. Switch off the current. Repeat the experiment using a small plotting compass instead of iron filings. Compare the directions of the compass needle to the patterns of iron filings on the card. Repeat the experiment with the direction of current reversed.

4.82 Magnetic field inside an open coil
See diagram 4.82: Open solenoid
Wind five evenly spaced turns of bell wire around a wooden cylinder. Slide the coil off the cylinder. Fit the cylinder into slots in a piece of cardboard so that the cardboard appears to cut the coil in half lengthways. Connect the coil to the terminals of a dry cell or lead cell accumulator or low voltage power supply using a car headlight bulb in series. Sprinkle iron filings evenly on the card. Switch on the current. Tap the card gently with the end of a pencil. The iron filings move into a pattern showing the magnetic field. Note the pattern inside the coil and outside the coil. Switch off the current. Repeat the experiment using a plotting compass instead of iron filings.

4.83 Electricity from a magnet and a coil
See diagram 4.83: Producing electricity with a magnet and a coil
Connect a coil of fifty turns of bell wire to a current detector. Use long connecting wires so that the coil, and the magnet are away from the compass in the current detector. Hold the horseshoe magnet or bar magnet in your left hand and the coil of bell wire in your right hand. Hold the coil vertically. Pass one pole of the magnet through the soil while observing the compass needle in the current detector. When the coil moves through the magnetic lines of force, an electric current moves through the circuit.

4.84 Make a simple electric motor
See diagram 4.84: Simple electric motor: 1. Horseshoe magnet, 2. Axle, 3. Commutator, 4. Coil, 5. Brass strip, 6. Electric motor with 3 coils, A Contact (brush) Aw Wire from contact to coil, B Contact (brush) Bw Wire from contact to coil
Fix a simple coil, mounted on an axle, between the poles of a horseshoe magnet. Two wires from the coil connect to the commutator. The commutator is a cylindrical insulator revolving on the axle with two strips of brass attached. The commutator rotates with the coil. Each brass strip is joined to one wire from the coil. Two carbon contacts, brushes, touch the side of the commutator and allow electric current to pass from the battery to the commutator. Electric current goes from the battery to brass strip A then along wire Aw, through the coil then back through wire Bw and brass strip B then back to the battery to complete the circuit. When the commutator and coil make one half turn, the current enters through brass strip B and returns through brass strip A, reversing the current in the coil. The electric motor runs more smoothly if more than one coil is used. This electric motor uses a permanent magnet but most electric motors use a field coil that forms a more powerful electromagnet.
Using Fleming's left hand rule, direction of thumb is thrust, first finger is magnetic field and second finger is current. In the diagram, side 7 to 8 of the coil has upward force on it and side 9 to 10 has downward force on it. So the coil turns until it is vertical and the brushes no longer touch the brass strips because of the gaps between them, and no current flows. However, due to inertia of the commutator, the coil keeps turning so side 7 to 8 is now on the right side and side 9 to 10 is on the left side. The brushes touch the brass strips again and the coil keeps turning clockwise.

4.85 Waves travel along a rope.
Attach coloured pieces of cloth to a rope at regular intervals. Tie one end of a rope to a support. Hold the other end so that the rope does not touch the ground. Make waves travel along the rope by moving the end of the rope up and down to make vertical waves, or moving left and right to make horizontal waves. Hold the rope still then strike the rope rhythmically with a stick to produce waves in the rope. Describe the motion of each coloured piece of cloth when a wave travels along a rope. Note the difference between the motion of one coloured piece of cloth and the piece next to it when the wave travels along the rope.

4.86 Make a ripple tank
See diagram 4.86: A ripple tank
1. The tank has with a glass bottom and it can hold water. Put the light source under the tank to see water ripples on the ceiling or put the light source over the tank to see water ripples on a sheet of paper below the tank. Use the tank in a dark place and where there is no vibration and no chance of anyone bumping into it. Adjust the depth to obtain only the required ripples. Fit sloping "beaches" of wire gauze around the edge. Note the circular pattern of ripples produced when a drop of water falls on the water in the tank. Use straight barriers and curved barriers with the height greater than the depth of the water and do not float.
2. Make a vibrator. Attach a piece of L-shaped thick wire to one end of a hacksaw blade. Clamp the other end of the hacksaw blade so that the end of the wire dips into the water. Pluck the end of the hacksaw blade and notice the circular waves formed in the water. For straight waves, attach a T-shape piece of tin to the end of the hacksaw blade.
3. Make an electric vibrator. Attach the L-shaped piece of wire or T-shape piece of tin to the armature of an electric bell.

4.88 Straight pulses
Make pulses by giving a cylindrical wooden rod a sharp push forward and back in the ripple tank. This motion produces continuous waves. The ripples are wider near the rod but sharper as they move away. The ripples are sharpest when the filament of the light bulb is parallel to them.

4.89 Reflection at a straight barrier
Note ripples hitting a straight barrier or the wall of the ripple tank:
1. circular pulses,
2. straight pulses hitting the wall at an angle of incidence smaller and greater than 45o.

4.90 Reflection at a curved barrier
Note ripples hitting a circular barrier:
1. on the outside,
2. on the inside.
Repeat the experiment with lens-shaped barriers.

4.91 Refraction of waves in a ripple tank
Put a plate of glass in the middle of the ripple tank to create a sloping depth. Note the distance between crests (wavelength) as the depth becomes more shallow. The wavelength is less and the velocity of the wave is also lower in the shallow water than it is in the deep water.

4.92 Diffraction in a ripple tank Ripple tank, wave tank
Diffraction occurs when a straight wave passes through a narrow gap. The waves spread at the edge of obstacles, e.g. edges of a gap, and curve in behind an isolated obstacle.
1. Note diffraction when a wave hits two barriers separated by a gap of about 1 cm or less. Place the barriers 5 cm from the source of vibration, the vibrating beam. Block off the outer end of the barriers with side barriers. Increase the width of the gap to about 10 cm and note less diffraction. Put weights on the barriers if they start to vibrate.
2. Repeat the experiment with two equally separated gaps. Increase the width of the gap and note less diffraction.

4.93 Sea wave patterns and sound wave patterns
See diagram 4.93: 1. Sea wave patterns 2. Sound wave patterns
1. A Ocean wave, B Boat wave, C Ripples, D Combination wave
2. A Pure note, B Different frequencies combined
The number of complete vibrations in one second is the frequency of a particular vibration. The way in which different sound frequencies combine is analogous to water waves. Ocean waves are longest, i.e. of low frequency. Let a small motorboat pass over these waves. The boat sends out its own waves, which have a higher frequency than ocean waves. Wind will make tiny ripples across the surface of the motorboat waves. The last ripples usually have an even higher frequency than the other two. These three vibrations can combine to form a pattern.

4.94 Wave patterns of a tuning fork
See diagram 4.94: Wave patterns of a tuning fork
A Fine wire, B Wax, C Smoked glass sheet on table, D Tuning fork not vibrating, E Base line, F Tuning fork vibrating
Use hot wax to attach a piece of fine wire to the prong of a tuning fork. Hold the fork rigidly by the handle and horizontally just above the table top. Use a candle to smoke a piece of glass. Lay the smoked glass under the prong with the fine wire bent to touch the glass. Start the tuning fork vibrations with the finger and move the glass along the table fast enough to make a wavy line on it. Repeat this experiment by moving the glass at different speeds and using different tuning forks. Note the markings on the tuning forks, e.g. "C", and compare the wave patterns.

4.95 Seeing and feeling vibrations that make sound waves
See diagram 4.97.1: Stretched rubber band
1. Stretch and pluck rubber bands and the strings of string instruments.
2. Hold a ruler on the edge of a desk with 15 cm extending over the edge and pluck it.
3. Put a drum on a desk and scatter puffed cereal grains or pieces of tissue paper or cork across the top. Strike the drum and watch the vibration.
4. Press the thumb and forefinger against the larynx and make a low-pitched sound with the voice. Feel the own sound vibration.
5. Hold a tuning fork loosely by the handle and strike the prongs against the edge of the desk. Note what you hear. Again, strike the prongs and quickly touch water in a pan with the tips of the prongs. The vibrating fork splatters the water.

4.97 Vibrating cans, string telephone
See diagram 4.97: String telephone
1. Punch a small hole in the bottom of a metal can. Pass a string or fishing line through the hole with its end tied in a big knot or tied to a match stick inside the can. Rub a resin on the string. Hold the can with one hand and keep the string tight with the fingers. Draw the fingers along the string. Sound comes from the metal can. Repeat the experiment by drawing the fingers along the string at different speeds. Note the different pitches of sound.
2. Drag a wet paper towel along the string. Some people say it sounds like a duck!
3. Simple string telephone
Cut the lids out of two used tin cans or use two plastic cups. Punch a small hole in the bottom of each can or cup. Pass cotton or fishing line or string through the holes with the end tied in a big knot or tied to a matchstick inside the can or cup. Pull the string tight. One person speaks into the can or cap while another person presses the other can or cup to the ear. Sound waves travel along the string to the bottom part of the can which acts as a diaphragm. Vibrations of the diaphragm transmit the sound waves through the air to the ear. Describe what happens when you speak into this telephone.

4.98 Sound waves travel through wood
To show that sound waves travel through wood, rest the ear against one end of a table top and gently tap the other end of the table with a ruler or pencil.

4.99 Materials that absorb sound
Test the sound absorbing properties of small pieces of material, e.g. rubber, sponge, felt. Place the piece of material on a wooden table top, strike a tuning fork, and bring the handle down on it. Then strike the tuning fork again and touch its handle on the wooden table top. Note which sound is louder.

4.100 Sound cannot travel through a vacuum
The speed of sound in air at 0oC = 331 ms -1. Use an aspirator or simple vacuum pump to pump the air from a large container or a bell container fitted with a spigot. Use a bicycle pump to make a simple vacuum pump. Open the pump and remove the piston. Unscrew the bolt that holds the leather washers then reverse the washers by turning them over. Replace the washers on the piston and reinsert the piston in the pump cylinder. Suspend a small bell from fine threads inside the container or bottle and shake the bell while the container fills with air. You can hear the bell ringing quite clearly. Use the aspirator or simple air pump to remove as much air as possible from the container. Shake the bell again. The sound of the bell is not as loud as before because sound cannot travel through a vacuum.

4.102 Low voltage light source
See diagram 28.199: Low voltage light source | See diagram 4.103: Low-voltage light source
Make a compact light source from any small, high intensity electric light bulb that has a short, straight filament, e.g. light bulbs used in car tail lamps. Use a small light source to make very sharp shadows with the light bulb filament end on. Cover the light source with a small drink-can. Darken the room. Punch 2 mm diameter holes in the drink-can on all sides. Blow smoke around the can to make the emerging rays visible. Make enough holes so that you can see clearly where the light comes from and in what direction it travels.

4.103 Luminescence
See 35.16 Luminescence, fluorescence, phosphorescence, triboluminescence, (Geology)
Luminescence is emission of light for any reason other than a rise in temperature, e.g. excited photons returning to a ground state. Chemiluminescence is luminescence resulting from a chemical change. However, the term phosphorescence is also used to describe a situation when the luminescence persists even though the exciting cause has been removed. Luminescence that does not persist when the exciting cause is removed is called fluorescence, e.g. a fluorescent light.

4.103.1 Candoluminescence
Candoluminescence is the light from heating substances with a flame to a high temperature so that some wavelengths are more than expected by blackbody emission at that temperature. This occurs in some transition metals and rare earth metal oxides, e.g. zinc oxide, cerium oxide, thorium dioxide.
Carl Auer, later Freiherr von Welsbach, 1858-1929, Austria, invented the Welsbach gas mantle containing thorium nitrate and cerium nitrate to increase light from gas lamps. Gas mantles are pieces of fabric soaked in metal oxides. They were used for gas lighting in homes, but nowadays are used only for gas pressure lamps for outdoors work and camping.

4.104 Luminance and illuminance, candela, candlepower, lumen, lux
See Luminous intensity, candela, cp
Luminous intensity, C, is a measure of the brightness of a light source, i.e. how much light emitted per second, and is measured in the candela, cd, formerly candle power. Luminance, L, measures the brightness of a surface in candela per square metre. A source of light measuring one candela emits one lumen of light, 1 lm.
Illuminance, or illumination, I, is a measure of the quantity of light falling on a surface at a distance from the light source, and is measured in lux, lx. Illuminance is directly proportional to luminous intensity, C, and inversely proportional to the square of the distance, d, from the light source, so I = C / d2, One lux is the illumination of one lumen per square metre. One lux is the brightness at one metre from 1 candela light source. Light meters, exposure meters, used in photography, measure illuminance in the unit lux.
See diagram 28.10.11: Projected Filament with Lens, thin lenses Turn on the light bulb. Move the light bulb to focus the image on the side wall. The focal lengths are marked on the lenses. Show the effect of aperture size on the sharpness on the focus by placing different sized stops in front of the lens.

4.105 Light travels in straight lines, pinhole magnifier
See diagram 28.105.1: Light travels in straight lines | See diagram 28.105.2: Pinhole camera | See diagram 4.105.2: Pinhole camera (no labels) | See diagram 28.105.3: Shadows | See diagram 4.105.3: Shadows (no labels)
1. Make a pinhole magnifier. Cut a very small hole through a piece of cardboard with a pin. Hold the cardboard very close to the eye in good light and look through the hole at some small print. The print appears larger and clearer because light rays pass through the small hole then spread out. The small hole functions like a camera shutter keeping out the extra light that would make the image blurred.
2. Look down on a tightly closed fist. Open the fist very slightly to let the smallest amount of light pass through. Look at some fine print through the fist. Move the fist up and down to get the best magnification.
3. Pierce a hole with the pin in the centre of a piece of cardboard. Hold it 10 cm in front of one eye. Hold the pin between the card and the eye. See an upside down image of the pin will be observed.
4. Make a pinhole in a sheet of aluminium foil. Hold the aluminium foil between a lighted candle and the wall. See the inverted image of the candle flame on the wall.
5. Hold the hole in the cardboard 3 cm from the eye. Keep the eyelid almost closed. See inverted images of the eyelashes. All objects will cast an upside down image on the retina when the eye is focussed on them. The brain interprets the upside down image as right side up.
6. Make a pinhole in the middle of one end of a rectangular box, e.g. a shoe box. Cut a window in the other end of the box and use adhesive tape to attach over it a screen made of greaseproof paper, lunch wrap paper, baking paper. Draw the letter T on a piece of thin white paper, or greaseproof paper using a marker pen. Attach the paper with the T drawn on it to the front of a light source. In a dark room, direct light from the light source towards the pinhole and, at the other end of the box, look at the image on the screen. The image of the T is inverted.

4.106 Reflection of beams of light
See diagram 28.106.1: Reflections | See diagram 28.106.2: Laws of reflection
Reflections: A Light source, B Comb, C Mirror
Laws of reflection: D Reflection in a plane mirror, E Eye, F Angle of incidence, G Angle of reflection
Laws of reflection: 1. The incident ray, reflected ray and the perpendicular normal, N, at the point of incidence all lie in the same plane. 2. The angle of incidence, i, = the angle of reflection, r.
Hold a comb so that the sun's rays shine through the teeth and fall on a piece of white cardboard laid flat on a table. Tilt the cardboard so that the beams of light are several centimetres long. Place a mirror held upright diagonally in the path. Note that the beams which strike the mirror reflect at the same angle. Turn the mirror and note the direction of reflected beams.

4.107 Smoke box to study light rays
See diagram 28.202: Smoke box to study light rays | See diagram 4.107: Ray tracing
A Smoke box, B Electric torch, about 1 metre from the smoke box, C Smoke, D Light beams, E White card
Make a wooden box 30 cm wide and 60 cm in length. Fit clear plastic or glass in the top and front of the box. Leave the back open and cover with a black cloth curtain. Hang this curtain in two sections, with a 10 cm overlap at the centre of the box. Paint the inside the box with black paint. Cut a window 10 cm high and 5 cm wide midway between the top and bottom of one end and 10 cm from the glass front. This window lets in light rays. You can cover the window with different kinds of openings cut from cardboard and fastened with drawing pins. Fix a piece of black cardboard with a 5 mm diameter hole over the window. Fill the box with smoke from a lighted incense stick or smouldering paper. Remove the smoke source and allow the apparatus to stand for 5 minutes to clear the smoke box of the heavier particles. The interior of the smoke box appears clear but still contains enough fine smoke particles to produce visible scattering of light rays.
Set up an electric torch or a projector 1 metre from the window. Focus the light down to a parallel beam and direct it at the holes in the window. The smoke makes the light rays in the box visible. Also, a laser may be mounted on a labjack and raised or lowered as required, or the light may be passed through a series of slides to produce multiple beams. Use the smoke box for ray tracing through optical elements, e.g. lenses, mirrors and prisms.

4.108 Reflection with a smoke box
See diagram 28.203: Reflection with a smoke box
A Reflected beam of light, B Mirror, C Light beam
Fill the smoke box with smoke. Shine the torch beam on the hole in the window. Hold a plane mirror inside the box and note the clearly defined rays after reflection from the mirror. The light rays reflect without scattering. Move the mirror to change the angle of reflection.

4.109 Mirror images, (inversion, lateral inversion)
See diagram 28.109.1: Lateral inversion | See diagram 28.204: Inversion
1. Write a name on a sheet of paper with a black pencil. Hold the paper up to the light with the writing away from you. Look at it with a mirror.
2. Write a name on a piece of carbon paper, carbon side up. Then read the underside of the sheet of paper. Look at it with a mirror.
3. Wear a heavily-printed T-shirt inside out. Look at yourself in the mirror.
4. Write a name on a piece of paper, but look at what you are writing on the paper only through a mirror. Some people can write in mirror images without using a mirror.
5. Look at the letters b, d, p, in a mirror, at the side of the letters, above or below the letters. What do the letters now read? Write a secret message in mirror writing.
6. Place a photograph of your face on the bench. Imagine a line that cuts the image of your face from top to bottom and exactly in half. Place a mirror vertically on the photograph with the back of the mirror on the imaginary line and the front of the mirror facing to the left. Move your head slightly to the left so that you can see your whole face, half the face from the photograph and half the face reflected in the mirror. Note whether you face is symmetrical and whether this composite image is the same as in the photograph.
7. Make an artificial mirror image. Fold a sheet of paper in half and paint a shape on one half. Fold the other half over the painted half and press down. Open the folded paper to see the mirror images.
8. Paint a design on the right side of your face. Look in a vertical mirror and notice that the design is on the left side of the face in the mirror. Place another vertical mirror so that its edge is touching the first mirror at an angle of about 120o. Position yourself so that you can see half your face in the first one mirror and the other half of your face in the second in the other mirror. The painted design is now on the right side of your face in the mirror.
9. Draw a 6-pointed star on a square piece of paper so that the points of the star almost touch the edges of the paper. Draw a second star 2 cm inside the first star. The area between the two stars is your star path. Place a barrier, e.g. a book, between you and the sheet of paper so that you cannot see the star path on the paper, but you can look over the book. Place a vertical mirror on the other side of the paper so that you can see the star path in the mirror. Hold a pencil vertically down on the star path. Move the
pencil around the star path until you come back to where you started. Note how long it takes you to move your pencil around the star path without running off it.

4.110 Ray box for beams of light
See diagram 28.205: Ray box | See diagram 4.110: Ray box: A Ray box, B Lamp C Lens, D Screens
This apparatus consists of two sides of an oblong box 22 × 6 cm with the lens placed at one end of the box. The box has no bottom, and in use rests on paper pinned to cardboard. The light source is a 12 V 24 watts, W, motor car lamp. The lamp holder has a sleeve of brass tubing just fitting into a hole in a wooden slide, which forms the top of the box. The groove in front of the lens is for screens and filters. A piece of card with a slit in it provides narrow rays, and a hair comb will give a bundle of rays. Adjust the position of the slider to form convergent, parallel or divergent beams. Do experiments with light rays using plane mirrors, glass blocks and prisms. A curved piece of tin will show a caustic curve. In experiments with lenses and in refraction, push down the lamp so that the light does not pass over the top of the obstacle. For optical experiments, in front of the lens use a card with a hole and cross wires.

4.111 Laws of reflection using a ray box
See diagram 28.206: Laws of reflection with a ray box | See diagram 4.111: Reflection with a ray box
A Laws of reflection with a ray box, B Lamp, C Lens, D Cork, E Mirror
Cut a vertical groove in a cork and fix a plane mirror in it by cutting a groove in the cork. Stand the mirror on the table. Place a piece of drawing paper in front of the mirror. Insert a board with a vertical slit in a ray box to make light rays travel along the paper surface and reach the mirror. Shine beams of light from the ray box along the paper and mark the path of the incident ray and the reflected ray with crosses. Join the crosses and continue the lines to the mirror. Remove the mirror. Draw the normal line at the intersection of
the above two lines. Measure the angle of incidence and the angle of reflection to see whether they equal.

4.112 Reflection from a concave mirror with a ray box
See diagram 28.207: Reflection from a concave mirror | See diagram 4.112: Reflection from a concave mirror
A Lamp, B Lens, C Concave mirror
Make a concave mirror from a fruit tin cut in half or a part of a metal ring. Measure the focal length of the mirror by directing a parallel beam of light on to it.

4.113 Reflection from a convex surface
See diagram 28.208: Reflection from a convex surface | See diagram 4.113: Reflection from a convex surface
A Convex mirror, B Lens, C Lamp
Use a convex mirror, e.g. a motor car wing mirror, side mirror, with the ray box and note the reflected rays of light. Compare its reflection with the reflection from a plane mirror and a concave mirror. In motor vehicles in Australia, Canada, India and USA, "Objects in the mirror are closer than they appear" is on the passenger side mirror because these convex mirrors makes objects appear smaller, e.g. another car behind in an adjacent lane. So the message is a warning against changing lane without warning.

4.114 Dispersion, spectrum with a ray box
See diagram 28.114: Dispersion with a triangular prism
See diagram 4.114: Dispersion with a triangular prism: A Normal, B White light, C Red, D violet
Dispersion occurs when light of different wavelengths is spread out by a prism into a spectrum
1. Use a glass prism to produce a spectrum from a parallel beam of light. Place a card with a narrow slit in front of the lens of a ray box. Use colour filters to suppress certain colours, e.g. use a transparent purple filter so that you see only red and blue lines on the screen.
2. Study light rays through a prism. Hold a glass prism in a parallel beam of light and note how the beam refracts. Rotate the prism on its axis. When white light splits into the colours of the spectrum, i.e. disperses, the violet light end of the spectrum refracts more than the red light. The refractive index of violet light is greater than the refractive index of red light. However, monochromatic light has only one colour and does not disperse.

4.115 Emission spectrum
If individual atoms of an element receive enough energy, they produce a characteristic line emission spectrum. Each element emits characteristic lines of radiation with specific wavelengths. Compounds contain more than one kind of atom, so they produce a band emission spectrum.

4.116 Incandescent lamp
Light bulb IEC Light Source 2 Wires 2.5V MES, "Scientrific" (commercial website)
Hot solids or liquids emit wavelengths of radiation depending on the temperature as a continuous spectrum. At lower temperatures they emit red wavelengths, so the metal appears to be "red hot". At higher temperatures, they emit the full visible spectrum as white light, so the metal appears to be "white hot" or "incandescent". The incandescent filament in an electric light globe, a filament lamp, is "white hot".

4.117 Absorption spectrum of sodium
1. When white light passes through a vapour of atoms, they absorb their characteristic wavelengths of light and reduce these wavelengths in the continuous spectrum emitted to produce a line absorption spectrum. White light from the sun travels through cooler elements surrounding it that absorb their characteristic wavelengths. The dark absorption lines in this line absorption spectrum, i.e. solar spectrum, identifies these elements, e.g. Helium.
2. Heat a wire coated in sodium chloride in a Bunsen burner flame and placed in front of a sodium light source. The sodium vapour from the heated wire appears as a black mist because of its absorption of the characteristic wavelengths of sodium.

4.120 Light rays through lenses
See diagram 28.120: Ray diagrams for lenses | See diagram 4.120: Ray diagrams to show the formation of images by lenses
A Real, inverted, diminished image, B Real, inverted same size image, C Real, inverted, magnified image, D Virtual, erect, magnified image, E Concave lens, virtual, erect, diminished image
Parallel rays of light that pass through a convex lens, converging lens, all pass through the principle focus, F. Parallel rays of light that pass through a concave, diverging lens, diverge as if coming from the principle focus, F. In the diagram, 1. to 4 are convex lenses that form real images when the object is more than one focal length from the lens.
1. Light rays come from a distant object,
2. The object is twice the focal length from the lens,
3. The object is between the focal length and twice the focal length from the lens,
4. The object is less than the focal length from the lens,
5. A concave always produces the same kind of image.

1. Take the lenses from an old pair of spectacles or used optical instruments, or purchase reading glass lenses and hand magnifiers. Cover the window of a smoke box with a piece of black cardboard with three holes punched in a vertical line. The holes should be the same distance apart, but the distance between the two outside holes should be a little less than the diameter of the lens. Arrange a torch supply parallel to light rays. Fill the box with smoke and hold a double convex lens in the path of the three beams of light so that the middle beam strikes the centre of the lens. Note the beams on the opposite side of the lens from the source of light. Repeat the experiment using a double concave lens.

4.120.1 Focal length of a convex lens
1. Attach a sheet of white paper on a wall opposite a bright window with the sun not visible because it is behind an outside object, e.g. a tree. The light rays passing through the window from the distant sun will be almost parallel. Hold a convex lens vertically about 5 cm from the paper the move it in a straight line towards the window until a clear image of the window appears on the white paper at a distance of the focal length of the lens.

4.121 Refraction in a smoke box
See diagram 28.212: Refraction in a smoke box | See diagram 4.121: Refraction in a smoke box
A Bottle of water, B Electric torch, C Light beam, D Refracted ray
1. Fasten a piece of black cardboard with a single hole in it 8 mm square over the window of the smoke box. Arrange a torch to shine a beam of light into the box. Fill a large, preferably rectangular, bottle with water and add a few drops of milk or a pinch of starch or flour to make the water cloudy. Cork the bottle. Fill the box with smoke. Hold the bottle at right angles to the beam of light and note the direction of the light through the water. Tilt the bottle at different angles to the beam of light and note how the path of light through the bottle changes.
2. Refraction is the change in direction of light as it crosses a boundary from one optical medium, e.g. glass, into another medium, e.g. air. Light bends towards the normal when entering a medium that is optically more dense. Light bends away from the normal when entering an optically less dense medium. Light paths are reversible for refraction. The incident ray, refracted ray, and normal to the boundary at the point of incidence, all lie in the same plane.

4.122 Refraction in water illusions, pool depth, bent stick, rising coin
See diagram 28.122.3: Rising coin | See diagram 4.122.3: Rising coin: 1. You can see A but not B, 2. You can see A and B as A1 and B1, 3. Air, 4. Water
See diagram 28.122.1: Stones in a swimming pool | See diagram 28.122.2: Bent stick
1. Drop three stones, (P1, P2, P3) in a flat bottom swimming pool. Drop P1 below you, P2 farther away and P3 at the far side. Look at the three stones from a position directly above P1. P1 appears to be at the greatest depth, P2 at lesser depth and P3 at still lesser depth. The bottom of the swimming pool filled with water appears curved when viewed from above. If the refractive index of water = 1.33, the apparent depth of the swimming pool looking straight down, normal view, = true depth / 1.33 = 3 /4 × true depth.
2. Place a stick in a tall container of water, so that part of the stick is above the surface. Note where the stick enters the water. The stick appears bent because the light rays refract as they pass from water to air. The image of each point on the stick below the water forms above its real position because of refraction at the air / water interface.
3. Put a coin in a non-transparent, short and thick cup on the table. Stand away, and arrange your line of vision so that you can just see a point A on the far side of the coin. Your view of the coin is almost shut out by the wall of the cup. Keep the position of your head unchanged while pouring water into the cup without moving the coin. As you pour in the water, the coin appears to rise, so you can now see the entire coin. The positions of A1 and B1 are the intersection of the backwards extensions of the refracted ray and the ray from A or B that is vertical to the surface of water and not refracted. The refracted ray from A is parallel to the refracted ray from B.
4. More than half fill a tall transparent glass with water. Insert a pencil so that the side of the pencil touches the right hand top of the glass and the lower end touches the left inner wall of the glass, but not the bottom. While looking down into the water, see the lower end of the pencil touching the wall and at the same time move your left finger from up and down along the wall of the glass until you think the finger points to the lower end of the pencil. Look through the side of the glass to see the actual position of the pencil. It is under your left finger. The position of the left finger is the position of the image of the end of the pencil.

4.123 Refractive index using real depth and apparent depth
See diagram 28.123: Real depth and apparent depth of glass
Place a block of glass on the table. Place a pin close to the side of the glass at O. The head of the pin may be seen from point A, at the edge of the glass opposite O. Place an inverted drawing pin at B on the glass. Adjust the position of B so that its point, coincides with the image of the pin at A seen through the glass. Measure the lengths of OA and A2. The plane CD with point A is the refraction plane of light, the refractive index from air into glass = AO / A2.

4.124 Refractive index using real depth and apparent depth, air to liquid
See diagram 28.124: Real depth and apparent depth of water is the curving of light around edge object and consequent spreading when it passes through a narrow gap. A single slit diffraction pattern differs from double slit interference.
1. Observe a vertical filament lamp slit formed by holding two finger together and looking through the narrow gap between the fingers.
2. Attach a pin at O to the bottom of a beaker with Plasticine (modelling clay). Place the beaker on the white paper on the table. Pour water into the beaker without disturbing the pin at O. Look down to see the image I of the pin at O through the liquid surface. Horizontally clamp another pin S to a stand near the beaker. Adjust the stand to make S at the same height as I. Mark the position of S on the outside of the beaker. Pour off the water in the beaker without disturbing the pin at O. Measure OL and IL, where L is a point on the surface of the water. Repeat the experiment with different heights of water. Calculate the reflective index from air into water = OL / IL.

4.125 Measure refractive index
Snell's law: sin i / sin r = n, a constant called the refractive index
Substance and refractive index (for liquids at 20oC): diamond 2.4173, flint glass 1.655, crown glass 1.517, ethanol 1.361, water 1.33299, carbon dioxide 1.00450, air 1.000293 vacuum 1.0
See diagram 4.125.1: Refraction: 1. Eye, 2. Torch, 3. Screen, 4. Incident ray, 5. Emergent ray
See diagram 4.125.2: Refractive index: A Incident ray, B Refracted ray, C Air, D Glass
See diagram 4.125.3 Pin against face of a glass slab: A Pin, B Pointer, C Slab
See diagram 28.125.1: Refraction | See diagram 28.125.2: Refractive index
1. Attach a black paper collar to the front of an electric torch. Prepare a screen with a 1 cm diameter hole, or use a CD-ROM disc as a screen. Hold the screen in front of the electric torch to limit the light beam to a narrow, horizontal beam. Put a rectangular container, e.g. a fish tank or transparent plastic box, on a sheet of white paper on the table. Draw a line on the white paper at right angles to the middle of the container, the normal. Draw another line at 45o to the first line. Fill the container with saltwater and add
drops or milk or fluorescein. Direct a beam of light along the 45o line into the container, the incident ray. Note the path of the beam of light through the water. Use smoke or chalk dust scattered in the air to make the beam of light visible in the air before entering and after leaving the container. Look through the end of the container, looking along the ray, to see that it is straight. The angle between the normal and the incident ray is the angle of incidence, i. The angle between the normal and the path of the light beam through the water is the angle of refraction, r. Refractive index = sin i / sin r. The beam of light leaving the container, after passing through the water, is the emergent ray. The incident ray and the emergent ray are parallel so there is lateral displacement between them. Lateral displacement depends on the breadth of the container, the angle of incidence and the refractive index of the air and the solution in the container.
2. Repeat the experiment by putting a rectangular slab of glass, or a rectangular plastic box contained full of a transparent solution, on white paper on the table. Draw the outline of the slab on the white paper. Place a pin, X. at the middle of the nearest side of the slab. Draw a line through X at 45o to the side of the slab. Look along the line and put two pins, A and B, on the line and two pins, C and D, in line with A and B on the opposite side of the slab. Put a pin, Y, where a line through DC meets the slab. Remove the slab and draw the normal at X (X1 to X2) and the normal at Y (Y1 to Y2). The path of the light ray is ABXYCD. Use a protractor to measure the angle of incidence AXX1 and the angle of refraction X2XY. Calculate the refractive index, sin AXX1 / sin X2XY. Check that AXX1 = DYY1, and X2XY = Y2YX. If refractive index of glass = 1.5, a glass slab viewed from the normal appears to be 1 / 1.5 = 2 / 3 of its true thickness.
3. Substance and refractive index (for liquids at 20oC): diamond 4.4173, flint glass 1.655, crown glass 1.517, ethanol 1.361, water 1.33299, carbon dioxide 1.00450, air 1.000293.
3. Put a pin against the far face of a glass slab. Hold a pointer down over the slab and move it until it is above the image of the pin, as seen through the slab. If the true thickness of the slab = T, and the apparent thickness = AT, i.e. the distance of the pointer from the front of the slab, then refractive index = T / TA.

4.126 Refraction from air to water
See diagram 28.215: Refraction in milky water | See diagram 4.1256: Refraction in milky water
Pour a few drops of milk into a glass of water to cloud the water. Punch a small hole in a piece of dark paper or cardboard. Place the glass in direct sunlight, and hold the card upright in front of the glass so that a beam of sunlight shines through the hole. First hold the card so that the hole is just below the water level. Note the direction of the beam in the water. Then raise the card until the beam strikes the surface of the water. Note the direction of the beam of light and experiment to find out how the angle at which the beam strikes the water affects the direction of the beam in the water.

4.127 Critical angle and total internal reflection
Critical angle is the angle of incidence in a more dense medium, which produces an angle of refraction of 90o in a less dense medium. Total internal reflection occurs when the critical angle of incidence is exceeded. Triangular prisms can change direction of light by total internal reflection if the angle of incidence > critical angle.
When a parallel beam of light from the lamp is aimed vertically upwards from beneath the cylindrical clear plastic trough containing water the beam is not deviated. It passes straight up.
If the beam is aimed up at a small angle of incidence, i, then both an internally reflected beam, R, and a refracted beam, T, can be seen. The beam, T, is passing from water to air, from a medium of higher optical density to lower optical density, so the refraction is away from the normal, angle r > angle i.
As the angle of incidence, i, is increased, r increases until the direction of the transmitted beam, T, approaches the direction of the surface of the water. When i reaches the critical angle the refracted ray just grazes the surface of the water, so angle of refraction becomes 90o. When i > critical angle there is no refracted beam because all the light is reflected as total internal reflection. There is a sudden increase in the intensity of the reflected beam as the angle of incidence increases beyond the critical angle. The critical angle is an angle of incidence in an optically more dense medium, which produces an angle of refraction of 90o in a less dense medium. When the critical angle of incidence is exceeded, there is no refracted light at all. Instead, all the light is totally internally reflected.

1. See diagram 28.127: Semicircular Plexiglas
Rotate a semicircular slab of Plexiglas with the light ray entering the exiting through the curved surface. Rotate the semicircular slab until the critical angle is reached and total internal reflection is obtained.

2.  See diagram 28.127.1: Candle behind fish tank
Put a short lighted candle behind a glass or plastic rectangular fish tank. Fill the fish tank with water to a level just above the wick. Look at right angles to the fish tank so that you can see the lighted candle directly opposite. Raise and lower the level of your eye above and below the level of the water. The top of the candle flame and the bottom of the candle flame around the wick are in one line. Move your head to the left parallel to the front glass of the fish tank. When your eye is above the water level, the top of the flame appears to move to the left. When your eye is below the water level, the bottom of the flame appear to move to the left. The angle between a line from the candle at right angles to the fish tank, the normal, and your line of sight, the incident ray, is increasing. For most glass, when this angle reaches about 43o, the critical angle, the incident ray cannot pass into the water, so the image disappears.

3. Return to the first position where you first looked at the candle directly opposite you. Lower your eye to the level of the bottom of the fish tank and look up at the bottom of the water surface. See the reflection of the lower part of the candle that you saw when your eye was just below the level of the water. Light from the candle up to the surface of the water is at an angle greater than the critical angle is reflected at the water surface, total internal reflection.

4. See diagram 28.127.2: Spoon in glass of water
Stand a spoon in a glass of water at the edge of the table. Look up from just below the table surface at the spoon pointing down towards you. The surface of the water acts like a mirror and so you see the reflection of the lower part of the spoon that is under water. However, you cannot see the upper part of the spoon above water.

5. "Pouring light"
IF-514 Tyndall's Experiment, total internal reflection, (28.6.1 Total internal reflection)
See diagram 28.216: "Pouring light"
In 1870, John Tyndall demonstrated to the Royal Society that light could be guided by a stream of falling water. So this experiment is often called "Tyndall's experiment". To show the behaviour of light in a constricted optical channel,  "pour" light from a drink-can. Remove the top of a drink-can. Punch a hole in the side of the drink near the bottom and close the hole with a stopper. Pour water into the drink-can until it is three quarters full. Put the drink-can next to a sink in a dark room. Hold an electric torch vertically down in the top of the drink-can so all the light shines down into the water. Remove the stopper and let the water pour into the sink. The light from the electric torch appears to pour out with the water. Most of the light cannot escape from the falling water because the critical angle is exceeded and it reflects off the water surface by total internal reflection. This principle is used for "light pipes", fibre optic cables and decorations using light shining up through a bunch of tubes.

6. Right angle prism
See diagram 28.127.3: Right angle prism
Shine a light into one of the two sides of a right angle reflecting prism. The light reflects off the hypotenuse and passes out through the other side. The light reflects because the angle of incidence at the hypotenuse is greater that the critical angle for crown glass, 43o. Reflecting prisms are used in binoculars, prismatic compasses and periscopes. Prisms allow you to see around corners!

4.128 Image with a convex lens
See diagram 28.217: Image with a magnifying glass
Darken all the windows in a room but one. Hold a convex lens (hand lens, magnifying glass) in the window and direct it at the scene outside. Bring a piece of white paper slowly near the lens until the image picture forms. Note the position of the image.

4.129 Magnifying power of a lens
See diagram 28.218: Magnifying power of a lens
Use a magnifying glass to get a clear image of the lines in an exercise book. Adjust the distance of the magnifying glass so that a line seen through the magnifying glass coincides with a line seen outside the magnifying glass. Compare the number of spaces seen outside the lens with a single space seen through the lens. The lens shown in the diagram magnifies three times. Linear magnification is the ratio of the size (height) of the image to that of the object or the image distance to the object distance. Magnification is the measure of enlargement or reduction of an object in an imaging optical system, e.g. X100. In astronomy it is the factor by which an image produced by an optical device increases the angular size of an object where magnification of a telescope = focal length of the telescope / focal length of the eyepiece.

4.129.1 Magnifiers, magnifying glass
Magnifier, X 2.5, "Scientrific", (commercial website) | Magnifier, on stand, "Scientrific", (commercial website) |
Magnifier, with lamp, "Scientrific", (commercial website) |
Magnifying glass, glass lens, magnification × 3, 75 mm diameter
Magnifying glass, bifocal, plastic lens, magnification 2 × and 6 ×, 75 mm diameter
Magnifying lens, hand lens, folded magnifier, magnification 10 ×

4.130 Water drop magnifier, lens
See diagram 28.1.17: Water lens. paper clip
1. Roll the end of a copper wire around a thick nail to make a loop. Cut the wire to leave a handle. Dip the loop in water then take it out so that the water in the loop is the shape of a convex lens. Look at the loop from the side to see the shape of the convex lens with the centre thicker than the edges. Use the water lens to look at a line in the palm of your hand. Move the lens towards and away from your hand to see the line become upright then inverted.
2. Very gently knock the loop so that the meniscus breaks then reforms to form a new water lens in the shape of a concave lens. Look at the loop from the side to see the shape of the concave lens with the centre thinner than the edges. Use the water lens to look at lines in the palm of your hand. Move the lens backwards and forwards.
3. Put a drop of water on a piece of clean glass. Observe the lines in the palm of your hand again. The drop of water acts as a magnifier.
4. Use needle nose pliers to bend the end of a "slide on" paper clip to form a loop. Dip the loop into a beaker of water then tap it against the side of the beaker to form a water lens inside the loop. The water lens could be a convex lens (widest in the middle) or a concave lens (thinnest in the middle). Examine the letter "e" with your water lens. Note whether the lens is a convex lens or concave lens. Dry the loop and try to make the other kind of lens.

4.131 Optical bench to study lenses
See diagram 28.219: Optical bench | See diagram 4.131: Optical bench: A screen B lens C light source D metre stick E blocks
An optical bench allows you to hold mirrors and lenses in position and to measure distances accurately with a metre scale. Use wooden or plastic blocks with grooves that just fit over the metre scale. Stick a pin into the centre of each block. Use strips of tin screwed to the side of the blocks to make lens holders. Attach a torch bulb to a block as a light source.

4.132 Colours of sunlight, rainbow
"Glass Prism", equilateral prism, rainbow spectrum, (commercial)
See diagram 28.220: Colours of sunlight
As the light passes from the air into the water droplet, it is refracted. White light is made of a wavelengths ranging from 400 to 700 nm. The index of refraction (n) is inversely proportional to the wavelength. Hence the index of refraction for the red wavelength (700
nm) is lower than the index of refraction for the violet wavelength (400 nm). Red light is bent less than the violet wavelength or the red light travels faster than the violet wavelength.

1. Simple spectrum. Pass white light, W, through a slit, S, then a lens, L, to obtain a pure spectrum on a screen, R, red to V, violet. N is the normal.
2. Darken a room into which the sun is shining. Drill a hole on a piece of thick cardboard. Cover the window of a room with a dark curtain, but leave a space for the piece of cardboard. Make sure that only one beam of light shines through the hole in the cardboard into the room. Hold a triangular glass prism in the beam of light so that it passes through the prism then reaches the opposite wall. Observe the coloured spectrum of sunlight produced through the prism on the opposite white wall.
3. Make the sunlight spectrum with a glass cup. Put a round glass cup without handle and colour on a windowsill. Fill it with water. Place a piece of white paper on the floor near the windowsill. Lift the cup so that you may see a rainbow or spectrum on the paper.

4.133 Electromagnetic radiation
See diagram 4.133: Electromagnetic radiation
f frequency in cycles per second, lambda wave length in metres, A gamma rays, B X-rays, C1 ultraviolet rays, C visible light, C2 infrared, D microwaves, E radar, F TV, H radio short waves, I radio waves
All electromagnetic radiation travels at the speed of light, c = 3 X 108 m s-1. The visible light spectrum has wavelengths 0.7 X 10-6 metres, red, to 0.4 X 10-6 metres, violet.
Sunlight is electromagnetic radiation in all ranges, Approx. wavelengths of radiation: gamma rays < 1 x 10-11 m, X-rays 1 x 10-11 to 1 x 10-8 m, ultraviolet rays 1 x 10-8 to 4 x 10-7 m, visible light rays 4 x 10-7 to 7 x 10-7 m, infrared rays 7 x 10-7 to 1 x 10-3 m, microwaves 1 x 10-3 to 1 x 10-1 TV and radio waves > 1 x 10-1m. Visible spectrum with approximate ranges of wavelengths in nanometres, nm (1 nanometre = 10-9 m): violet 390 to 425 nm, indigo 425 to 445 nm, blue 445 to 500 nm, green 500 to 575 nm, yellow 575 to 590 nm, orange 590 to 620 nm, red 620 to 780 nm. The velocity of light in a vacuum, c = 3.00 X 108 m / second, but less in transparent materials, e.g. air 2.99 X 108 m / second, water 2.25 X 108 m / second, glass 2.00 X 108 m / second. The microwave region of the electromagnetic spectrum is from wavelengths 1 m to 1 mm. In Australia, microwave ovens operate at 12 cm wavelength.

4.134 Spectroscope, diffraction grating
See diagram 4.134.1: Spectroscope: A Eye, B Hole covered outside by diffraction grating, C Slit, D Razor blades, E Vertical filament lamp
See diagram 4.134.2: Diffraction grating: A sunlight, r red, v violet
A diffraction grating is a piece of plastic or glass with many opaque parallel lines rules on it, e.g. 100 lines per mm, 300 lines per mm, 1000 per mm, 13,500 lines per inch. When light rays enter the spectroscope, they are separated, according to different wavelengths, into a spectrum or spectra and produce an interference pattern are sharpened to appear as bright lines of reinforcement (maxima). Each element has its own characteristic bright lines on its spectrum so the spectroscope is used for chemical analysis. Spectroscopes are also used in astronomy to determine the elements in the sun and stars because it can produce separated line images for light sources with similar wavelengths. The spectroscope invented by Joseph von Fraunhofer in 1820 used fine parallel wires.
1. Make a diffraction grating by drawing evenly-spaced clear black lines on a white card. Then take a high quality black and white photograph using a camera stand. Use the negative for a diffraction grating. However, you can also purchase cheap diffraction gratings as novelty spectacles, sometimes called "rainbow glasses".
2. Cut a 2 cm diameter round hole at one end of a cardboard shoe box. Attach a diffraction grating across the hole on the inside of the box. Note the direction of the slit on the grating. In the opposite side of the box, cut a 0.5 cm X 2.5 cm slit opposite the diffraction grating, with the longer side horizontal. Attach two razor blades to the outside of the slit, almost edge to edge, to form a very narrow vertical slit. Place a 12 V vertical filament lamp, e.g. a neon lamp or argon lamp, in front of the slit. Adjust the distance between the two razor blades so that you may see clear linear spectrums when you look through the round hole. Use the diffraction grating and a sharp source of light to see the order of colours in the spectrum. ROYGBIV, represents red, orange, yellow, green, blue, indigo and violet. Note the bright lines in spectra produced by fluorescent mercury lamps and neon signs.
3. Hold a feather near your eye and observe a burning candle far from you. Adjust the distance of feather from your eye until you see four X-shaped colour bands. You may also see two blue and two red bands in each of the four bands.
4. Stretch nylon gauze or a woman's fine scarf tightly and observe a burning candle through it. See colour stripes appearing in the direction of the fibres. Different weaving and different shapes of small holes will affect different shape of the stripes. You may see an X-shaped diffraction pattern through some types of nylon gauze.
5. Make a spectrum without a prism. Set a tray of water in bright sunlight. Lean a rectangular pocket mirror against an inside edge with the lower part immersed in the water. Adjust the mirror so that a spectrum appears on the wall.
6. Pass light through a spherical flask of water and view the rainbow on a screen placed between the light and the flask.

4.135 Infrared rays source
See diagram 28.223: Infrared rays: A Heat lamp, B Visible light, C Iodine solution, D Infrared rays, E Burning black paper.
See diagram 4.135.1: Spectrum pic | See diagram 4.135.2: IR Spectrum pic (University of Melbourne)
1. Iodine dissolved in alcohol gives a filter transmitting in the IR but absorbing in the visible. To produce infrared radiation, use a heat lamp for treating muscular ailments. Fix the infrared lamp on the table so that it shines horizontally on the bulb of a large flask of water. The flask acts as a lens. Hold your hand between the lamp and the flask to feel the heat. Move a piece of black paper on the other side of the flask to find the focal point. Add iodine solution to the water and shake the flask to make the iodine solution uniform. Place the flask back at the original position. Hold a piece of cotton wool soaked in methylated spirit at the focal point. It starts to burn. Iodine solution stops visible light but allows the longer infrared wavelengths to pass through. Infrared radiation is invisible electromagnetic radiation of wavelength between about 0.7 micrometers, (0.7 m), and 1 millimetre, (1 mm), i.e. between the limit of the red end of the visible spectrum and the shortest microwaves. All objects above 0 K, including humans, absorb and radiate infrared radiation. Infrared radiation is used in medical photography and treatment, in astronomy and in photography in fog. Infrared radiation can be detected by a Golay cell detector that contains xenon gas.
2. Show that electromagnetic radiation extends beyond the visible into the infrared and its equivalence with heat radiation. A normal colour spectrum is produced with the aid of the slit and slide projector and the prism. Rotating the prism will bring different sections of the spectrum into the entrance pupil of the thermopile. Maximum reading is obtained just passed the red end of the spectrum. This experiment requires that the infrared filter is removed from the slide projector. Plastic slides will melt.
3. Set up a slide projector to display a normal spectrum on the screen. Remove the IR filter and place a 2-3 mm slit in the slide carriage. Focus a digital movie camera on the image and compare the images in normal mode and night vision mode. The CCD elements are sensitive to the infra red and normally an IR filter is used to block the IR. In night vision mode this filter is swung out of the way, allowing the infra red to be displayed.

4.136 Ultraviolet light source
See diagram 28.224: Ultraviolet light source
1. Attach two lamp holders to insulating material and fasten it to the bottom of a cardboard carton with the top removed. Fix two argon lamps into the lamp holders and connect the lamps in parallel without leaving any bare wire exposed. Cut a notch in the side or end of the box for the electrical lead cord. Invert the box cut a peephole to allow viewing without direct eye exposure to the ultraviolet light. Ultraviolet light may cause serious damage to the eyes. However, you can observe different objects in "black light" by placing the cardboard box over the objects, turning on the switch to the power source and observing the objects through the peep hole. Objects that glow under ultraviolet light include clothing dyed with fluorescent dyes, e.g. socks and ties, soap powders containing an "optical brightener", e.g. "Bluo", and white clothes washed in these powders, fluorescent paints and lacquers, fluorescent chalk, some minerals, e.g. willemite, fluorites, opals and sphalerites.
2. Use an argon lamp as an ultraviolet light source to display fluorescence. Mount an argon lamp in a light proof box and cut a peephole in the box for viewing. Be careful! Avoid direct eye exposure to the ultraviolet light, which may damage the eyes. To note different objects in black light, put the box over the objects and turn on the argon lamp. Clothes may contain fluorescent dyes, e.g. bright socks. Ultraviolet rays in ordinary sunlight cause fluorescent dye to glow. Soap powders may contain a brightener. White clothes washed in these powders fluoresce in the ultraviolet radiation from the sun or from an argon light bulb. Fluorescent paints, lacquers and chalk are also available. Some minerals fluoresce in ultraviolet light, e.g. ilmenite, opal, sphalerite and some fluorites.
3. Collect objects that glow under ultraviolet light. Ultraviolet light is used for bank note testing, in hospitals and in fluorescent watches. Ultraviolet radiation is light rays invisible to the human eye, of wavelengths from about 4 × 10-7 to 5 × 10-9 metres, where the X-ray range begins. Ultraviolet radiation causes sunburn and the formation of vitamin D in the skin. Ultraviolet rays are strongly germicidal and may be produced artificially by mercury vapour lamps for therapeutic use. The radiation may be detected with ordinary photographic plates or films.

4.137 Colours of soap films
Make a strong soap solution as used for blowing soap bubbles. Fill a flat dish with the solution then dip a cup into the solution until a soap film forms across the cup. Hold this in a strong light so that the light reflects from the film. Note the colours. Tilt the cup to make the film vertical, and note the changes in the colour pattern as the film becomes thinner towards the top. The colours seen in thin films come from the interference of the light waves reflected from the front and the back of the film.

4.138 Colours of oil films
1. Add black ink to a flat dish filled with water. Put the dish in a window where light from the sky is very bright but not in direct sunlight. Look into the water so that light from the sky reflects to your eye. While looking at the water, place a drop of oil on the nearest surface at the edge of the dish. Note a brilliant rainbow of colours flashing away from you towards the opposite edge. Blow on the surface to see a change in the colours. Interference of white light results in spectral coloured fringes.
2. Add two drops of clear nail varnish to a bowl of water. Dip black paper in the water and leave it to dry. Look at the paper in sunlight from different angles and see the rainbows form as light is dispersed by the layers of nail varnish.

4.139 Colours of transparent objects, colour filters
See diagram 28.227: Colour filters
Study colour filters. Observe the coloured light that passes through a transparent object and the colour of the transparent object. Prepare some transparent objects with different colours, e.g. coloured glass, coloured cellophane. Roll a cylinder with a piece of white paper and fix it vertically above a piece of white paper on the table. Put the transparent objects on the cylinder under sunlight or white light so that light shines down through the transparent object. Observe the colour of the paper on the table and compare it with the colour of the transparent object. The colours are the same. Transparent objects absorb some colours and some colours to pass through them. They have colour because of the colours they transmit and that they absorb all other colours. Water has high transparency. It absorbs some light in the infrared and ultraviolet regions of the spectrum but transmits the visible radiation necessary for photosynthesis. The colour of a transparent object is a mixture of those wavelengths that it transmits. The colour of an opaque object has a colour due to the mixture of wavelengths it reflects, the others being absorbed. The diffused light is the colour of light that the object absorbs less. The nature of the surface of an object can affect the direct reflection of different coloured light. If the ratio of reflection to certain colour light is greater than that of other colour light, the object may appear the colour of this colour light. A white opaque body, or a colourless transparent body reflects or transmits all wavelengths in the same proportion as they occur in white light. A polished silver surface may reflect 93% of the white light incident upon it and white paper may reflect 80%, depending on the nature of the surface and the angle of incidence.

4.140 Colours of opaque objects
1. Note the colour of a piece of red cloth in white light or sunlight. In a dark room, note the colour of the same piece of red cloth in red, blue, green, and yellow. The red cloth appears black unless placed in light of the same colour or in white light or sunlight. Opaque objects have colour because of the light they reflect. In white light or sunlight they absorb the other colours of the spectrum. Repeat the experiment with a piece of white cloth. White objects may reflect any colour. Repeat the experiment with a piece of
black cloth. Black objects absorb all colours and do not reflect any colour. Repeat the experiment with coloured illustrations from a magazine. In white light or sunlight, remember the colour of each part, e.g. red flowers and green leaves, then compare its colour under the coloured light.
2. Note the colour of dry sand. Add water to the sand and note any change of colour. Dry sand is composed of pieces of quartz that reflect light in all directions so that the sand appears almost white. When sand is wet, the layer of water on each quartz grain reflects back some light at the air water surface, so the sand appears darker in colour.

4.141 Mix coloured pigments, blue and yellow chalk
Use a piece of blue chalk and a piece of yellow chalk. Crush them and mix them evenly. The mixture will be green. The green here is not pure. It is between the colour of yellow and green in the spectrum. The colour of yellow absorbs all colours except yellow and green. The colour of blue absorbs all colours except blue and green. So only yellow, blue and green are reflected. However, the yellow and blue absorb each other, so the light reflected into your eyes is only the green colour. Mixed pigments reflect the common colour for all the pigments in the mixture and subtract all the other colours. Repeat the experiment with water colours with the same density.

4.142 Rotate colour discs
Newton's colour wheel, "Scientrific", "Scientrific", (commercial website)
See diagram 28.230: Rotate colour discs
1. Mix coloured lights by using water colours painted on discs of cardboard. Paint a yellow "egg yolk" on one side of a 10 cm disc, and a blue "yolk" on the other side. Suspend the disc between loops of string. Twist the loops then pull outwards to make the disc spin. The resulting colour is nearly white.
2. Paint radial segments alternately red and green. Note the resulting mixture of red and green lights reflected to the eye by spinning the disc on a string.
3. Divide a white disc into seven segments. Paint each segment with one of the seven colours of the visible spectrum, (violet, indigo, blue, green, yellow, orange, red). Spin the disc rapidly, e.g. attached to an electric motor. The disc appears nearly white, depending on the purity of the colours. This disc is called Newton's disc or Newton's colour wheel.

4.143 Mix coloured lights
Shine red, blue and green lights on a white screen so that the colours overlap. Red overlaps with blue to produce magenta. Blue overlaps with green to produce turquoise, blue-green. Green overlaps with red to produce yellow. In the centre, red, blue and green overlap to produce white, so red, blue and green are called the primary colours. Magenta, turquoise and yellow are called the secondary colours. For colour photography, each primary colour is processed separately by its layer of light sensitive emulsion. For colour television, the primary colours are separated by the camera and added together again in the television set. The "primary colours" of an artist are red, blue and yellow, not red, blue and green, because artists use pigments, not coloured lights.

4.144 Colours of the blue sky and the sunset
See diagram 28.144: Colours of the blue sky and the sunset
When light passes through the atmosphere more of the shorter waves from the blue end of the spectrum are scattered by gas molecules in the air and small dust particles than the longer waves from the red end of the spectrum. So the blue light scatters in all directions and the sky appears blue in all directions. So the light from a low sun at sunrise and sunset contains mostly waves from the red end of the spectrum. During the day, not much light is scattered light from a high sun.

1. Observe ripples of water passing through upright reeds and note that shorter wavelength ripple are scattered more by passing through the reeds than longer wavelength ripples.
2. Shine a narrow beam of light through a fish tank or a large beaker filled with water. Add drops of milk or powdered milk or acidified sodium thiosulfate solution while stirring until you can see the beam shining through the water. Look at the beam both from the side and from the end, where the beam shines out of the container. Viewed from the side, the beam appears blue. Viewed parallel to the direction of the beam, the beam appears orange-red or yellow. See the colour of the beam change from blue-white to orange-yellow along the length of the beam. Let the light project onto a white card at the end of the tank. The beam spreads so it is not so narrow as at the source of light. Particles in the milk scatter the light and so allow you can see the beam from the side. Blue light is scattered much more than orange light or red light, so we see more blue light from the side. Orange light and red light are scattered less so we see them at the end. The shorter wavelength blue light has a greater refractive index so it bends more than longer wavelength red light with a smaller refractive index. Similarly, atmospheric gases smaller than one wavelength scatter blue light, so the sky appears blue. This phenomenon is called Rayleigh scattering. The sun is white hot but it appears orange-red because the white light from it has lost some blue light. When the sun is on the horizon, its light takes a longer path through the atmosphere to your eyes than when directly overhead. So at sunset most of the blue light is lost by scattering leaving the orange-red light, i.e. white light minus blue light. Only the longer wavelengths reach the eyes. If there were no scattering, and all the light from the sun travelled straight to the earth, if not looking at the sun, the sky would look dark as it does at night. Large particles, e.g. dust, smoke, and pollen, scatter light without breaking white light into component colours. This is called Mie scattering. It is the cause of the whiteness of clouds, mist,
milk, latex paint and the white glare around the sun and moon during a mist. The sun has the same colour as a black body at 5780 K.
3. Place a lens from Polaroid sunglasses between the light source and the fish tank. Hold the lens vertically and turn it while another person observes the beam from above and another person observes the beam from the side. When the person above observes a bright beam, the person at the side observes a dim beam, and vice versa. This is the same effect when look through two parallel sun glass lenses and you turn one of the lenses. At a certain position no light, or very little light, passes through both lenses. So the scattering in the fish tank polarizes the light. Light emitted by the sun, by a lamp in the classroom, or by a candle flame is unpolarized light. Electromagnetic light waves from the sun or an electric lamp come from electric charges vibrating in many directions perpendicular to the direction of the light beam. Sunglasses include a Polaroid material that absorbs light vibrating horizontally and so reduces glare. So the light reaching your eyes is polarized light.

4.145 Colours of the sea
The sea appears blue because it absorbs all of the wavelengths of sunlight except the short blue wavelength. The oxygen content of water molecules absorbs the red end of the spectrum. Blue light is scattered in water in all directions to cause the blue oceans. Similarly at the North and South polar regions the ice and icebergs appear blue. The blue colour changes if the sea contains phytoplankton, suspended sediments, and dissolved organic chemicals as in the seas in the temperate regions.
4.145.1 Colours of water
1. Observe from above the water in the deeper end an indoor swimming pool with white ties and illuminated with white light. It appears blue because the red component of the light reflecting from the bottom of the pool is absorbed. It looks less blue at the shallow end of the pool. Thus, as white light travels through water, the red / orange / yellow components of light get absorbed by the water and cause the water molecules to vibrate, while high frequency (blue) photons continue to travel through the water.  The net result is a slight increase in the water temperature (molecular vibrations translate directly into temperature), and the white light  turns blue as it travels deep through the water. The presence of dissolves salts, sediments and algae may affect the colour of water.
2.  Poke a hole in the snow with a 1 m long stick or ski pole. The hole looks darker and blue. Ice containing many small air bubbles appears white.
4.146 Balance with a metre stick, stationary meeting point, centre of mass, centre of gravity
See diagram 8.146: Stationary meeting point | See diagram 4.146: Uniform rod | See diagram 4.146.1: Metre stick
A body acts as if its mass is concentrated at a single point, the centre of mass. Gravity acts through the same point, the centre of gravity. If a vertical line through the centre of gravity of an object does not pass through its base, the object falls over. An object, e.g. a motor car, will not roll over easily if it has a low centre of gravity and a wide base. The centre of gravity of a metre stick or uniform rod is in the centre. If two fingers support the rod and one finger moves towards the centre of gravity the rod begins to tip towards that finger to increase the weight and increase the force of friction. The other finger feels less wight and has less friction so the rod easily slides above it.
1. Support a metre stick or uniform rod over your two index fingers so that each finger is exactly 1 cm from the end. The weight on the fingers feels exactly the same. Keep the left finger in place but slowly move the right finger towards the centre until it is half way between the centre and the end. The metre stick feels heavier on the right finger than on the left finger. Move the fingers together while keeping the metre stick balanced. As your left finger moves towards the right finger, the metre stick feels heavier on it. The weight on each finger feels about the same when the two fingers move together to be just each side of the centre of gravity.
2. Repeat the experiment by moving one finger quickly and the other finger slowly. Maintain the ruler in balance while moving the fingers. If the metre stick remains horizontal, the two fingers always meet at the centre of the metre stick.
3. Repeat the experiment using two round smooth pencils on a level table instead of fingers. Move the right pencil towards the middle of the rod while holding the left pencil in place. As the right pencil approaches the middle of the rod the pencils have the same distance to the ends of the rod.
4. Repeat the experiment by hanging your hat on one end of the metre stick. Note the new position of the centre of gravity.
5. Repeat the experiment with a broom to find its centre of gravity.
6. Slide two kitchen scales under a loaded beam. Note the scale readings of the moving and stationary scales change in the same way that your fingers feel change in weight under the metre stick.
7. Put an empty drink-can on a rough wooden board. Raise one end of the board until the drink-can falls over. At that angle, a vertical line through the centre of gravity of the drink-can passes outside its base.
8. Stand still then raise your right arm sideways. Nothing happens. Raise your right leg sideways. If your upper body moves to the left, your centre of gravity remains over your left foot so you remain stable. If you keep your upper body rigid, your centre of gravity moves to the right and is no longer over your left foot, so you fall over.

4.147 Ball bearings fall together
See diagram 8.234: Simultaneous fall | See diagram 14.2.4: Spring-loaded device
1. A spring loaded device drops one ball and projects the other horizontally.
2. Two balls simultaneously dropped and projected horizontally hit the floor together. Drop one billiard ball and shoot another out simultaneous. One ball is projected horizontally as another is dropped simultaneously. Instructor rolls a ball off the hand while walking at a constant velocity.
3. Use two clothes-pegs, a pair of ball bearings and a wide rubber band. Fix the band lengthways around one peg. Then open the peg and force a ball against the tension of the rubber band between the prongs of the peg. Grip the other ball with the second peg. Hold the pegs side-by-side, pointing away horizontally above the floor. Squeeze both pegs at once. At the same moment, one ball begins to fall vertically, and the other is shot forwards. Note what happens by looking and listening very carefully. Repeat the experiment from different heights and with a tighter rubber band. If the experiment is done correctly, while the ball bearings land in different places they strike the ground simultaneously.

4.148 Acceleration of marbles down an incline
Use a 3 m plank of wood with a groove down the centre. Incline the plank so that marbles can roll down the groove. Arrange small tin flags hung from wires so that the marbles hit them and make "clinks" sounds. Put the flags at regular intervals, e.g. 25, 50, 75, 100 cm, from the end of the plank. Roll a marble down the groove and listen to the time intervals between "clink" sounds. The time intervals between the "clinks" reduce as the ball rolls down the incline.
Arrange the flags so that the clinks occur at equal intervals of time. Measure the distance between the flags. The distance between the flags increases down the incline in the ratio 1:3:5:7:9.

4.150 Coupled pendulums
See diagram 4.150: Coupled pendulums
Fill two same size bottles with water, add stoppers and suspend the bottles with same size string as pendulums from a rod. Hold one bottle still, start the other bottle swinging, then release the first bottle. Soon the swinging pendulum slows, and the other pendulum takes up the swing.

4.153 Three-holes can, 3-hole can, a vase with three holes, spouting cylinder, Mariotte's flask
See diagram 4.153: Three-holes can
Punch three identical holes in the side of a plastic drink bottle at 1/4, 1/2 and 3/4 of the height, but offset so that the streams of water do not interfere with each other. Plug the holes then fill the bottle with water. Put the bottle on a table with a sink draining top. Attach a tube to a tap to keep a constant head of water when you remove the plugs. Remove the plugs. Note the speed the water through the three holes. Feel the water with your finger as it comes out of the hole. The fastest water stream is through the lowest hole. Note how much water passes through each hole in the same period. Note the path of the water streams. Draw a diagram of the three water streams showing the distances travelled by each stream to the table top. Diagram 4.153.2 is incorrect, although it occurs in some textbooks. Diagram 4.153.1 is correct. Water from the middle hole hits the table at the greatest distance from the bottle, d2. Water from the bottom and top holes both hit the table at the same lesser distance from the bottle, d1. The greater the depth, the greater the pressure. Liquid pressure increases with depth.