School Science Lessons
UNPhysics1a, Colour, gas discharge tubes, light
sources, reflection, refraction, sound, waves
Please send comments to: J.Elfick@uq.edu.au
Updated: 2012-01-28 SP
Table of contents
4.132 Colour
4.147 Gas discharge tubes
4.103 Light sources, producing light
4.106 Reflection of light at flat surfaces, plane
mirrors
4.114 Refraction of light at flat surfaces, magnifiers
4.93 Sound
4.85 Waves
4.132 Colour
4.117 Absorption spectrum
4.132 Colours of sunlight, rainbow
4.137 Colours of soap films
4.138 Colours of oil films
4.139 Colours of transparent objects, colour filters
4.140 Colours of opaque objects
4.144 Colours of the blue sky and the sunset
4.145 Colours of the sea
4.114 Spectrum with a ray box, dispersion
4.115 Emission spectrum
4.118 Fluorescent lamp
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
4.136 Ultraviolet light source
4.103 Light sources, producing
light
4.117 Absorption spectrum
4.115 Emission spectrum
4.118 Fluorescent lamp
4.116 Incandescent lamp
4.135 Infrared rays source
4.120 Light rays through lenses
4.105 Light travels in straight lines, pinhole
magnifier
4.103 Low voltage light source
4.104 Luminance and illuminance
4.106 Reflection of light
at flat surfaces, plane mirrors
4.106 Reflecting beams of light
4.111 Laws of reflection with 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
4.108 Reflection with a smoke box
4.107 Smoke box to study light rays
4.114 Refraction of light
at flat surfaces, magnifiers
4.146 Refraction of light
4.127 Critical angle and total internal reflection,
"pouring" light
4.128 Image with a convex lens, magnifying glass
4.120 Light rays through lenses
4.129 Magnifying power of a lens
4.129.1 Magnifying glasses
4.125 Measure refractive index
4.131 Optical bench to study 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
4.126 Refraction from air to water
4.130 Water drop magnifier, water lens
4.93 Sound
4.96 Kazoo tube, comb kazoo, goose horn tube
4.99 Materials that absorb sound
4.95 Seeing and feeling vibrations that make sound
waves
4.93 Sound waves patterns
4.98 Sound waves travel through wood
4.97 Vibrating cans, string telephone
4.94 Wave patterns of a tuning fork
4.85 Waves
4.87 Circular pulses
4.92 Diffraction in a ripple tank
4.90 Reflection at a curved barrier
4.89 Reflection at a straight barrier
4.91 Refraction of waves
4.86 Ripple tank
4.88 Straight pulses
4.85 Waves travel along a rope
Cold cathode tubes, discharge tubes: 7.4.0
Current through an electrolyte: 29.2.1.3
Electrophorus: 31.1.8.1 See 2.
Radiation hazards: 2.12 See 2.
Simple discharge tube: 38.1.12
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 Ripple tank
See diagram 25.183: 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:
1. on the outside,
2. 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.
4.93 Sound waves patterns
See diagram 26.190: 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 26.191: Wave patterns of a tuning
fork | See diagram 4.94: Wave patterns of a tuning
fork (no labels)
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
Stretch and pluck rubber bands and available string instruments. Hold
a ruler on the edge of a desk with 15 cm extending over the edge and pluck
it. Put a drum on a desk and scatter puffed cereal grains across the top.
Strike the drum and watch the cereal grains vibrate. Press your thumb and
forefinger against your larynx and make a low pitched sound with your
voice. Feel your own sound vibration. Hold a tuning fork loosely by the
handle and strike the prongs against the edge of the desk. What do you hear?
Strike the prongs again, and this time quickly touch water in a pan with
the tips of the prongs. What happens? The vibrating fork splatters the water.
4.96 Kazoo tube,
comb kazoo, goose horn tube
1. Kazoo tube
See diagram 26.193: Kazoo tube
Use a large cardboard tube, e.g. a post office mailing tube. Cut a
square of waxed paper large enough to be wrapped around the end of the
tube. Secure the waxed paper 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.
2. Comb kazoo
Hold a straight hair comb with the teeth pointing downwards. Cut out
a piece of waxed paper twice the area of the comb. Fold the piece od waxed
paper into two and place it over the comb so that each side is covered.
While holding the waxed paper against the comb use it to touch your lips
while you make a "oo, oo, oo" sound. The waxed paper vibrate to change
the original sound and cause a tingling sensation in your lips.
2.1 Thunder bag
Cut out a same size pieces of thin cardboard and paper. Fold both squares
in half diagonally. Put the piece of paper inside the piece of cardboard
and glue the edges together.
3. Goose horn tube
See diagram 26.193: 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 people can also put the end of the tube
with the tab inside the mouth and produce a sound by blowing into the tube.
4.97 Vibrating cans, string
telephone
See diagram 26.195: String telephone | See diagram 4.97.1: Make string sounds
1. Punch a small hole in the bottom of a metal drink-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. Repeat the experiment with soft styrofoam cup instead of a metal
drink-can and tooth picks that be easily poked through the styrofoam. The
sound quality exceeds that when using tin cups, plastic cups and paper cups.
3. 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. 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.
4.98 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 rubber, sponge,
felt, and other materials. Place the piece to be tested on a wooden table
top, strike a tuning fork, and bring its handle down on a piece of material.
Then strike the tuning fork again and touch its handle on the bare wood
top. Which is louder? Try each material.
.
4.103 Low voltage light source
See diagram 28.199: 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 6.3.1.7: 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.
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 Reflecting beams of
light
See diagram 28.106.1: Reflections | See diagram 28.106.2: 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 Smoke box to study light
rays
See diagram 28.202: 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 28.203: 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, (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 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.
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 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
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 28.206: 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 28.207: 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 28.208: Reflection from a convex
surface
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
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 Spectrum with a ray
box, dispersion
See diagram 28.114: Dispersion with a triangular
prism | See diagram 4.114: Dispersion with a triangular
prism (no labels)
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
See diagram 28.133: Incandescent 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
See diagram 28.199: Fluorescent lamp, low
voltage light source
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.
4.120 Light rays through lenses
See diagram 28.120: Ray diagrams for lenses
| See diagram 4.120: Ray diagrams for lenses (no labels)
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 28.212: 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 28.122.1: Stones in a swimming
pool | See diagram 28.122.2: Bent stick | See diagram 28.122.3: Rising coin
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 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 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
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 28.125.1: Refraction | See diagram 28.125.2: Refractive index | See diagram 4.125.2: Refractive index (no labels)
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 4.126: 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 28.127.1: Candle behind fish
tank | See diagram 28.127.2: Spoon in glass of
water | See diagram 28.216: "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 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 Magnifying glasses
Magnifying glass, glass lens, magnification X 3, 75 mm diameter
Magnifying glass, bifocal, plastic lens, magnification 2 X and 6 X, 75 mm diameter
Magnifying lens, hand lens, folded magnifier, magnification 10 X
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
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
See diagram 28.220: Colours of sunlight
Order online: Glass
Prism, equilateral prism, rainbow spectrum
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.
4.135 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
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 µm), 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.
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, turn on the
switch to the power source and observe 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. "Buo", 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 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 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
which 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
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.
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
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 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. 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.
4.146 Refraction of light
Light travels more slowly when it moves from a less dense medium to
a more dense medium. When the direction of the light is at right angles
to the boundary, the wavelength decreases but the frequency remains the
same. If light through air then glass then the refractive index of the glass
= speed of light through the air / speed of light through the glass, so here,
refractive index must be < 1. When the direction of the light is not
at right to the boundary, the light changes speed and direction because
the light that first meets the boundary slows before the light that later
meets the boundary. The change of speed and direction is called diffraction.
Deep and shallow water cause water waves to behave as if moving between
two different media because the side of the wave to first meet the shallow
water slows before the rest of the wave. Tsunamis occur when huge waves
slow and rapidly increase in height when entering shallow water.
4.147 Gas discharge tubes
The gas discharge tube contains gas at low pressure. The electric field
from the potential difference between the cathode and anode causes ions
and free electrons to accelerate towards the electrodes and collide with
particles to excite them and create more ions. The decay of the excited
particles releases characteristic light. In an evacuated discharge tube
electrons from the cathode can be deflected by the application of electric
or magnetic fields so electrons must be charged particles. The light from
discharge tubes containing pure elements can be analysed using a spectrophotometer
into the lines of different colours emitted. This information can lead
to identification of elements in a sample. Using a hydrogen discharge tube
the frequencies of visible light, UV radiation and infra-red radiation
can be measured to explain the energy transition level for the electron
in the hydrogen atom in kJ mol-1. The flow of electric charge
through gas occurs in fluorescent light bulbs and lightning.