Physics experiments
Please send comments to: J.Elfick@uq.edu.au
Updated: 2008-07-16
Table of contents
4.85.0 Make waves
4.93.0 Sound
4.103.0
Producing light
4.106.0
Reflection
4.114.0
Refraction
4.132.0
Colour
4.145.0 Balances
4.147.0 Gravity
4.85.0 Make waves
4.85
Waves travel along a rope
4.86
Make a ripple tank
4.87
Circular pulses
4.88
Straight pulses
4.89
Reflection at a straight
barrier
4.90
Reflection at a curved
barrier
4.91
Refraction of waves
4.92 Diffraction in a ripple tank
4.93.0 Sound
4.93
Sound wave patterns
4.94
Wave patterns of a tuning
fork
4.95
Seeing and feeling
vibrations that make sound waves
4.96
A bell from a spoon
4.97
Vibrating cans, string
telephone
4.97.1 Goose horn tube
4.97.2 Kazoo tube
4.97.3 Comb kazoo
4.98
Sound waves travel through
wood
4.99
Materials that absorb sound
4.100
Sound cannot travel through
a vacuum
4.101
The ear and hearing
4.102
The voice and speaking
3.12 String telephone (primary)
4.103.0 Producing light
4.103
Sources of light
4.104
Luminance and illuminance
4.105 Light travels in straight
lines, pinhole magnifier
4.106.0 Reflection
4.106
Reflecting beams of light
4.107
Make a smoke box to study
light rays
4.108
Reflection with a smoke box
4.109
Reversed writing
4.110
Make a ray box for beams of
light
4.111
Laws of reflection with a ray
box
4.112
Reflection from a concave
mirror with a ray box
4.113
Reflection from a convex
surface
4.114.0 Refraction
4.114
Study the spectrum with a
ray box
4.115
Emission spectrum
4.116 Incandescent lamp
4.117 Absorption spectrum
4.118 Fluorescent lamp
4.119
Diffraction of light
4.120
Light rays through lenses
4.121
Refraction in a smoke box
4.122
Refraction in water
4.123 Refractive index
using real depth and apparent depth
4.124 Refractive index using real depth and
apparent depth, air to liquid
4.125
Measure refractive index
4.126
Refraction from air to water
4.127
Critical angle and total
internal reflection, "pouring" light
4.128
Image with a convex lens, magnifying glass
4.129
Magnifying power of a lens
4.130 Water lens
4.131 Optical bench to study lenses
4.132.0 Colour
4.132
Colour of sunlight
4.133
Electromagnetic radiation
4.134
Colour experiments
4.135
Use infrared rays
4.136
Use ultraviolet light
4.137
Colours in a soap film
4.138
Colours in an oil film
4.139
Colour of transparent objects, colour filters
4.140
Colour of opaque objects
4.141
Mix coloured pigments, blue
and yellow chalk
4.142
Rotate colour discs
4.143 Mix coloured lights
4.144 Colours of the blue sky
and the sunset
4.144.1 Colour of the sea
4.145.0
Balances
4.145 Balance with a see-saw
(teeter-totter)
4.146 Balance with a metre stick,
stationary
meeting point, centre of mass, centre of gravity
1.21 Balanced mobile (primary)
1.3 Weighing devices (primary)
4.147.0
Gravity
4.147 Ball bearings fall together
4.148 Acceleration of marbles down
an incline
4.149 Simple pendulum
4.150 Coupled pendulums
4.151 Time a falling body
4.152 Paths of projectiles, free fall
4.153 Three-holes can
4.154 Falling washers on a
string
1.41 Falling parachutes (Primary)
7.109 Gravitational potential energy
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: 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.
4.87 Circular pulses
1. Touch the water with 1. a finger 2. a pencil point 3. a drop
of water from an eye dropper. Note a single circular ripple in the
middle of the tank. Make several such ripples one after the other.
2. Touch the water simultaneously in two places. Note the circular
ripples crossing over each other.
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 a. on the outside, b. on the
inside. Repeat the experiment with lens-shaped barriers.
4.91 Refraction of waves
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
1. Note diffraction when a wave hits two barriers separated by a gap
of 2 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 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 Sound wave patterns
See diagram 4.93: Sound wave patterns
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
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: Make string
sounds
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.
4.96 A bell from a spoon
See diagram 4.96: See and feeling vibrations
that make sound waves
Tie the middle of one metre of string around a fork. Tie each end of
the string around the index fingers. Press the ends of the string into
the ears with the fingertips and let the spoon hang down loosely.
Note the different sounds when the spoon swings and hits different
objects, e.g. wooden table, glass
window, iron pot. Hit the spoon with
another spoon and hear a chime like a bell. Sound waves travel along
the string to the ears.
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. Drag a wet
paper towel along the string or rub your wet fingers along the string.
Some people say it sounds like a duck or a chicken.
2. 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.
3. Cut
the lids out of two used tin cans or use cylindrical cardboard food
cartons with a metal lids. Punch a very small hole in the
bottom of each tin can and push the ends of several metres of thin
cotton
string through the holes. Attach matchsticks or a small nut to the ends
of
the string inside the tin cans or tie a big knot in the ends. If you
cannot punch a hole through the bottoms of the tin cans attach the ends
of the string with adhesive plaster or glue. Pull the string tight and
talk and listen to
the other person. The speaker holds the tin can tightly to the face and
speaks into it. The listener person holds the other tin can
tightly over the ear and listens. The string telephone does not work
around corners because the string must not touch any object. The
speaker should first speak very loudly and then speak very softly.
Sound waves from the speaker's voice cause the bottom of the tin can to
vibrate. This vibration then moves along the tight string and then into
the bottom of the listener's tin can. The bottoms of
the tin cans act as diaphragms. Vibrations of the diaphragm of the
listener's tin can transmit
the sound waves through the air to the listener's ear.
4.97.1 Goose
horn tube
See diagram 4.97: Goose horn tube
Cut a 10 cm X 10 cm square of thin cardboard with a 1 cm X 1 cm tab at
one corner. Roll the cardboard into a tube leaving the end where the
tab is until last. Bend the tab over the end of the tube. The tab
must completely cover the open end of the tube so you may have to roll
the tube again more tightly. Use adhesive tape to secure the tube. Suck
on the end of the tube away from the tab. The tab makes a noise like a
goose when it vibrates against the end of the tube. Some peolple can
also put the end of the tube with the tab inside the mouth and produce
a sounnd by blowing into the tube.
4.97.2 Kazoo
tube
See diagram 4.97: Kazoo tube
Use a large cardboard tube, e.g. a post office mailing tube. Cut a
sqyare of waxed paper large enough to be wrapped around theione end of
the tube. Secure the waxed paped with a thick rubber band.Make a hole
in the side of the tube about 4 cm from the covered end. Press the open
end of the tube around your mouth and make a humming noise or say
"doing, doing". The quality of the sound is changed by the vibrating
membrane so this instrument can be called a membranophone. Repeat the
experiment with kitchen aluminium foil instead of waxed paper.
4.97.3 Comb
kazoo
Hold a straight hair comb with the teeth pointing downwards. Cut out a
piec of waxed apaper twice the area of the comb. Fold the piece od
waxed paper into two and place it over the comb so that eaxh side is
covered. While holding the waxed paper against the combuse it to touch
your lips while you nmake a "oo, oo, oo" sound. The waxed paper
vibrate to change the original sound and cause a tingling sensation in
your lips.
Thunder bag
Cut out asame size pieces of thin cardboard and paper. Fold both
squares in half diagonally. Put the pieco of paper inside the piece of
cardboard and glue the edges together.
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.
Repeat the experiment by putting a loud ticking watch into a vacuum
flask.
4.101 The ear and hearing
See diagram 4.101: Vertical section of human
ear
1. Eardrum, 2. Incus, stapes, malleus, 3. Auditory nerve, 4. ear
canal, 5.
Middle ear, 6. Inner ear, circulatory canals, cochlea, 7. Eustachian
tube
1. A sound wave is a longitudinal wave so it consists of
alternating compression and rarefaction, i.e. particles closer
together and farther apart. A sound wave can pass through the ear canal
of the outer ear to reach the sensitive eardrum, tympanic
membrane, and causing it vibrate at the same frequency. The eardrum is
attached to the bones of the middle ear, the ossicles. The hammer
(malleus) connects the eardrum to the anvil (incus) that connects to
the stirrup (stapes) that connects to the oval window of the middle
ear. These bones transmit vibration from the eardrum to fluid in the
cochlea, the portion of the inner ear responsible for hearing. It
looks like a snail's shell. These vibrations in the inner ear cause
nerve
impulses to be transmitted to the brain by the auditory nerve. The
bones of the
skull can also transmit vibrations. You hear a sound if
the waves reach the cochlea by either route. When a sound reaches our
two ears, you can distinguish the direction from which it comes. If it
comes from straight ahead, the vibrations reach both ears
simultaneously and with the same strength. However, if the source of
sound is on one side, one ear is farther away from it and receives the
sound waves less strongly and with a slight delay. The eardrum must be
protected. A perforated eardrum can lead to serious infection. Never
use a bobby pin or cotton buds to clean the ear. When the ear canal
gets
blocked with wax, treat it with medical ear drops. Do not hit anyone on
the ear! The eardrum can become
perforated if the outer ear is hit with the open
palm of the hand.
2. Besides the cochlea, the
inner ear also contains three small semicircular canals to maintain
balance. Movement of fluid in the semicircular canals sends messages to
the brain about the speed of rotation of the head and the direction of
movement of
the head, e.g. nodding or looking behind. Spinning the whole body
causes giddiness, vertigo. To experience extreme vertigo, mark a cross
on the floor, bend the body at right angles, rotate the body with one
eye looking down at the cross. Be careful! This movement may
cause nausea.
3. The Eustachian tube extends from the middle ear
to the nasopharynx. Usually it is closed. It can open to let air pass
and equalize the pressure between the middle
ear and the atmosphere, causing a small "pop" sound. This happens
during change of height in an aircraft or during mountain travel.
People, and especially babies, with eustachian tubes blocked with mucus
experience pain when an aircraft changes height. To balance the
pressure in the middle ear with the outside pressure, hold the nose
shut and blow softly or blow the nose or chew chewing gum . During
flight, the air pressure in a commercial aircraft is usually regulated
to the pressure at 1500 to 200 metres above ground.
4.102 The voice and speaking
See diagram 4.102: Voice
1. vocal cords, 2. epiglottis, 3. during ordinary breathing, 4. during
speaking, 5. larynx
Mouth, teeth, tongue, throat and lungs are all used in the production
of the voice. The sound is produced by vibrations of two thin sheets of
membrane, the vocal cords, stretched across the sound chamber,
the larynx. The larynx is the upper end of the windpipe and is near the
base of the tongue. A trapdoor of cartilage, the epiglottis,
automatically drops down over the larynx when you swallow, so that no
food can enter the windpipe. When the vocal cords are stretched by the
contraction of certain muscles in the throat, a narrow slit forms
between them. It is when the air is forced through this narrow slit
that the cords are forced to vibrate. This sets the air vibrating in
the windpipe, lungs, mouth and nasal cavities.
4.103 Sources of light
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.
4.104
Luminance and illuminance
See also 7.14: candela
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.
4.105 Light travels in straight lines, pinhole
magnifier
See diagram 4.105.1: Light travels in
straight
lines | See diagram 4.105.2:
Pinhole camera | See diagram 4.105.3:
Shadows
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 Reflecting beams of light
See diagram 4.106.1: Reflections | See
diagram 4.106.2: Laws
of
reflection
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 Make a smoke box to study light rays
See diagram 4.107: Smoke box to study light rays
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 smouldering paper. 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.
4.108 Reflection with a smoke box
See diagram 4.108: Reflection with a smoke box
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
See diagram 4.109.1: Lateral inversion | See
diagram 4.109.2: Inversion
1. Produce mirror images
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.
2. 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.
3. Look at the letters b, d, p, in a mirror 1. at the side of the
letters 2. above or below the letters. What do the letters now read?
Write a secret message in mirror writing.
4.110 Make a ray box for beams of light
See diagram 4.110: Ray box
This apparatus consists of two sides of an oblong box 22 X 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 with a ray box
See diagram 4.111: Laws of reflection with a
ray box
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 4.112: Reflection from a 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 4.113: Reflection from a convex
surface
Use a convex mirror, e.g. a motor car wing 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.
4.114 Study
the spectrum with a ray box, dispersion
See diagram 4.114: Dispersion with a
triangular prism
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
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
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.
4.118 Fluorescent lamp
Materials may emit light and other radiation when illuminated by
higher frequency radiation or by streams of electrons. Electron tubes
contain gas under reduced pressure that allow movement of
electrons between electrodes, e.g. the now obsolete thermionic
valve. A fluorescent lamp is a gas discharge tube
containing mercury vapour at low pressure. It emits light because the
inside of the tube is coated with a fluorescent substance,
phosphor. Electric current passes through the mercury
vapour and produces ultraviolet radiation that hits the phosphor and is
converted to light. In a neon tube, the gas discharge tube contains
neon
gas at low pressure that glows red. In a cathode ray tube, or in an
X-ray
tube, electrons from a heated cathode hit a fluorescent screen to
produce light.
4.119 Diffraction of light
Diffraction is the curving of light around the edges of objects and the
consequent spreading of light when it passes through narrow gaps.
Single slit diffraction pattern is different from double slit
interference. Look at a vertical filament lamp through the slit formed
by holding two
fingers together.
4.120 Light rays through lenses
See diagram 4.120: Ray diagrams for lenses
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.
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.
4.121 Refraction in a smoke box
See diagram 4.121: Refraction in a smoke box
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, depth of a swimming
pool, bent stick illusion, rising coin illusion
See diagram 4.122.1: Stones
in a swimming pool | See diagram 4.122.2:
Bent
stick | See diagram 4.122.3:
Rising
coin
1. Drop three stones, P1, P2, and 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 X 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 pencil.
4.123 Refractive index
using real depth and apparent depth
See diagram 4.123: 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 4.124: Real depth and apparent
depth of water
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
See diagram 4.125.1: Refraction | See diagram 4.125.2: 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.
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 4.126: 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
See diagram 4.127.1:
Candle
behind fish tank | See diagram 4.127.2:
Spoon in
glass of water | See diagram 4.127.3:
"Pouring light"
1. 1. 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. 2.
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.
2. 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.
3. "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.
4. 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 4.128: 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 4.129: 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.
4.130 Water lens
See diagram 4.130: Water lens
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.
4.131 Optical bench to study lenses
See diagram 4.131: Optical bench
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 Colour of sunlight
See diagram 4.132: Colour of sunlight
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
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 4.99 X 108
m / second, water 4.25 X 108 m / second, glass 4.00 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 Colour experiments
See diagram 4.134.1: Spectroscope | See diagram 4.134.2: Diffraction grating
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 4.5 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 Use infrared rays
See diagram 4.135: Infrared rays
A Heat lamp, B Visible light, C Iodine solution, D Infrared rays, E
Burning black paper
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 and 1 millimetre, 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.
4.136 Use ultraviolet light
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. Avoid direct eye exposure to the
ultraviolet light that 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 fluoresces 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. 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 X
10-7 to 5 X 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 in a soap film
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 in an oil film
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 Colour of transparent objects, colour filters
See diagram 4.139: 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.
4.140 Colour 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
See diagram 4.142: Colour mixtures
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.
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 4.144: Colours of the blue sky and
the sunset
1. 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 than the longer waves from the red end of the spectrum. 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. 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
yellow-orange 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.
4.144.1 Colour 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. The reflected blue light
is scatterered in all directions.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 Balance
with a see-saw (teeter-totter)
See diagram 4.145: Balance with a see-saw
1. Make a see-saw with 3 m board and a sawhorse for a fulcrum. Use two
students of equal weight. Sit at either end of the board so that they
balance. Measure the distance from the fulcrum, balance point, to
each student. Multiply the distance by the weight of the student.
2. Select a heavier student and a lighter student. Tell them to sit
on the board so that they balance. Measure the distance from the
fulcrum to each student. Multiply the distance by the student's weight.
3. Select a heavier student, weight m1, and a lighter
student, weight m2. Sit on the board so that they balance.
Measure the distance from the fulcrum to each student, d1
and d2. Multiply the distance by the student's weight. You
will discover that m1d1 = m2d2. d.
Select a heavier student, weight m1, and two lighter
students, weight m2 and m2. Sit on the board so
that they balance. Measure the distance from the fulcrum to each
student. Multiply the distance by the student's weight. Add the
products for the two lighter students.
m1d1 = m2d2. m1d1
= m2d2 + m3d3.
4.146 Balance with a metre stick, stationary
meeting point, centre of mass, centre of gravity
See diagram 4.146:
Stationary meeting point
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.
1. Support a metre stick over your two index fingers. Place the finger
of your right hand under one end of a metre stick. Place the finger of
your left hand half way between the centre and the other 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 by hanging
your hat on one end of the metre
stick. Note the new position of the centre of gravity.
4. Repeat the
experiment with a broom to find its centre of gravity.
5. 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.
6. 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.
7. 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 4.147: Ball bearings fall together
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.
4.149 Simple pendulum
1. Tie a 2 m string to a heavy object. Swing
the pendulum bob through
a small angle, < 20o, between the pendulum and a vertical
line at the bottom of the swing. Count the number of swings per minute.
Each swing starts and finishes when the bob passes the rest position in
the same direction. Swing the pendulum bob through a smaller arc. Count
the number of swings per minute. The size of the arc does not affect
the time of vibration of a pendulum. The angular amplitude does not
affect the period if the pendulum bob swings through a small angle.
2. Use the same 2 m pendulum, but change the weight of the bob. Count
the number of swings per minute. The change in weight does not affect
the time of oscillation of the pendulum. The mass of the pendulum bob
does not affect the period.
3. Change the length of the pendulum to two metres. Count the number
of swings per minute. The number of swings per minute increases. The
length of the pendulum affects the time of oscillation of the
pendulum.
4. Measure the time taken for 25 oscillations through the same small
angle for pendulum length, l = 0.25, 0.50, 0.75 and 1.00 m.
Calculate the time for one swing, T, for each length. Assume the
square roots of length, l, are 0.5, 0.71, 0.87 and 1 m. Draw a graph to
plot T against sqrt length, l. If the graph line is a straight line,
then T is proportional to the square root of length l. The period,
T, is the time taken to complete on one complete oscillation,
forwards and backwards. Period, T = 1 / frequency, f (expressed in
Hertz, Hz, cycles per second, cps). The period, T, of a simple pendulum
depends on its length, l, and the acceleration because of gravity
(acceleration of free fall) g = 9.8 m / sec2. T = 2 X pi X
sqrt (l / g). If the period, T = 1 second, T = 2 X pi X sqrt (l / g) l
= g X T2 / 4 X pi2, so length, l = 0.25 m. A
simple pendulum, length 25 cm, swings through one complete cycle every
second. Simple harmonic motion refers to movement of equal distance
each side of a central point with acceleration towards the central
point and proportional to the distance to it. The bob moves with simple
harmonic motion because the force on the bob at any point is
proportional to its displacement from its mean position and is directed
towards it.
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.
4.151 Time a falling body
See diagram 4.151.1: Throw up and fall down |
See
diagram 4.151.2: Ticker timer
1. Time a falling body. Throw a ball as high as you can. Use a
stopwatch to measure
1. from when it leaves your hand to when
the ball reaches the greatest height and stops rising,
2. from when
the ball starts to fall and reaches the height of your hand. Time up
equals time down.
2. Time a falling body with a ticker timer. Attach a weight to a strip
of paper tape. Pass the tape between the
armature of an electric bell and a pad of carbon paper. Release the
paper tape so that the weight falls and drags the paper after it. The
end of the arm of the timer hits the carbon paper against the tape and
makes marks on it at equal time intervals. Measure the distance between
the marks.
3. Make a ticker timer from an electric bell mechanism. Remove the
clapper and extend the armature by soldering a strip of metal to it. At
the end of this extension, drill a hole to fit a small round head
screw. Fix the screw head downwards to act as a marking hammer. Fasten
the mechanism to a wooden base. Fix a 3 cm diameter disc of a carbon
paper disc to the base with a drawing pin. The drawing pin holds the
disc loosely at the centre so that the disc can rotate to expose a new
surface as the tape passes under it. Attach staples to the base to
guide the path of the ticker tape. If the extension to the armature
strikes the paper too hard, the timing may be uneven.
4.152 Paths of projectiles, free fall
See diagram 4.152: Path of a projectile | See also 2.0.5: Conic sections, parabola | See also 2.0.6: Parabola equation
1. The apparatus is used to show that the vertical and
horizontal velocities of a projectile are independent. The projectile
is a metal ball, e.g. a ball-bearing. The target is a metal drink-can,
suspended by an electromagnet. The circuit includes two
wires attached to a copper gate at the entrance to the cardboard tube.
When the circuit is closed, the drink-can is kept in position by the
electromagnet. Sight along path p1 then blows
the ball up the cardboard tube. The ball hits the copper gate to open
the circuit and let the drink-can fall. The ball travels through path
p2 and hits the drink-can target in mid-air.
2. The following experiment can be applied to different projectiles,
e.g.
golf balls, cannon balls, darts, discus, shot put, slingshot, catapult
and long jumper. If a projectile has initial horizontal velocity before
it starts falling, its trajectory is a parabola, e.g. ball rolling off
a table. The only force on a projectile is gravity.
Throw a ball vertically as high as you can. Note the time between
when the ball leaves your hand and the ball stops rising. Also, note
the time between when the ball stops rising and the ball descends to
the height of your hand. The times are the same. Throw the ball up at
different angles to the horizontal and note the times taken for rising
and descent.
3. Hold ball 1 just over the edge of the table so that it can hit the
floor when you drop it. Put an identical ball 2 in the middle of the
table and use a stick to push it steadily towards the edge of the
table. When ball 2 passes the edge of the table immediately drop ball
1. Ball 2 has original horizontal velocity but ball 1 has no horizontal
velocity. However, both balls fall and land on the floor simultaneously
because the acceleration because of gravity is the same, whatever their
state of motion. Both balls are projectiles.
4. A body in free fall descends height, h = 1/2 gt2,
where t = time and g = acceleration because of gravity, 9.8 m / sec2.
The time interval for both balls to reach the floor, t = sqrt (2h
/ g). If the table is 1 m high and velocity of ball B is 5 m / sec, the
time of fall, t = sqrt (2 X 1) / 98 = 0.45 sec. The distance ball B
travelled before reaching the floor, d = vt = 5 X 0.45 = 4.25 m.
Find your "hang time", i.e. the time you are off the ground when you
make
a vertical jump. Hold a piece of adhesive tape between your thumb and
finger, jump up next to a wall, and leave the sticky tape on the wall
at the top of your jump. Use the formula: t = sqrt (2h
/ g), where h is the distance from the sticky tape to the floor. How
high can you jump? An American basketball player can do a 1.25 metres
vertical jump!
5. Use a bow and arrow by pulling the bowstring back a certain
distance and pointing the arrow at a certain angle to the
horizontal before releasing the bowstring. Note the maximum height of
the arrow and the distance it travels before hitting the ground. Repeat
the experiment by pulling the bowstring back the same distance but
change the angle to the horizontal. Find the angle to the horizontal
where the arrow attains the greatest height and longest distance before
hitting the ground.
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 5.1.1a is
incorrect, although it occurs in some textbooks. Diagram 5.1.1b 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.
4.154 Falling washers on a string
See diagram 4.154: Falling washers on a string
1. Tie an iron washer to one end of a 2 m string. Tie another six
washers every 30 cm along the string. Stand on a chair, hold the end of
the string with no washer attached and let the string hang down. Let
the bottom washer be 30 cm above the floor. Release the string and
listen to the sound of the washers hitting the floor. The sound
intervals are not the same, they get smaller as the washers hit the
floor.
2. To repeat the experiment, tie an iron washer to one end of a
4 m string, then tie on other washers 15 cm, 45 cm. 75 cm, 105 cm
and 135 cm apart. So the distances of the washers from the end of
the string are 15 cm, 60 cm, 135 cm, 240 cm and 375 cm. Stand on a
chair, hold the end of the string with no washer attached and let the
string hang down. Let the bottom washer be 30 cm above the floor.
Release the string and listen to the sound of the washers hitting the
floor. The sound intervals are the same. (0.175 seconds). The
acceleration because of gravity (acceleration of free fall) g = 9.8 m
/ sec2. For a free falling body, h = gt2 / 2,
so
as time increases the speed of a free falling body is faster and
faster. Using the formula h = gt2/2, the falling distances
of a free falling body at 1, 2, 3, 4, 5 seconds are 0.5g, 2g, 4.5g, 8g,
14.5g. The falling distances every second are 0.5g, 2g - 0.5 g = 1.5g,
4.5g - 2 g = 4.5g, 8 g - 4.5 g = 3.5g, 14.5 g - 8 g = 4.5, i.e. the
ratio is 0.5 : 1.5 : 4.5 : 3.5 : 4 = 1 : 3 : 5 : 7 : 9. For similar
falling objects separated by the ratio of distances 1: 3: 5: 7 : 9, the
time intervals of falling are equal.