School Science Lessons
UNPhysics1a Colour, gas discharge tubes, light sources, reflection,
refraction, sound, waves
Please send comments to: J.Elfick@uq.edu.au
2012-05-05c SPP
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
4.120.1 Focal length of a convex lens
4.128 Image with a convex lens, magnifying glass
4.120 Light rays through lenses
4.129 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 an
"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 × 10 cm square of thin cardboard with a 1 cm ×
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: 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?
Test 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 × 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.
4.120.1 Focal length of a convex lens
Attach a sheet of white paper on a wall opposite a bright window with the
sun not visible because it is
behind an outside object, e.g. a tree. The
light rays passing through the window from the distant sun will be
almost
parallel. Hold a convex lens vertically about 5 cm from the paper the move
it in a straight line
towards the window until a clear image of the window
appears on the white paper at a distance of the
focal length of the lens.
4.121 Refraction in a smoke
box
See diagram 28.212: Refraction in a smoke box
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 × 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 × 3, 75 mm diameter
Magnifying glass, bifocal, plastic lens, magnification 2 × and
6 ×, 75 mm diameter
Magnifying lens, hand lens, folded magnifier, magnification 10 ×
4.130 Water drop magnifier,
lens
See diagram 28.1.17: Water lens. paper clip
1. Roll the end of a copper wire around a thick nail to make a loop.
Cut the wire to leave a handle. Dip
the loop in water then take it out so
that the water in the loop is the shape of a convex lens. Look at the
loop
from the side to see the shape of the convex lens with the centre thicker
than the edges. Use the
water lens to look at a line in the palm of your
hand. Move the lens towards and away from your hand to
see the line become
upright then inverted.
2. Very gently knock the loop so that the meniscus breaks then reforms
to form a new water lens in the
shape of a concave lens. Look at the loop
from the side to see the shape of the concave lens with the
centre thinner
than the edges. Use the water lens to look at lines in the palm of your
hand. Move the lens
backwards and forwards.
3. Put a drop of water on a piece of clean glass. Observe the lines
in the palm of your hand again. The
drop of water acts as a magnifier.
4. Use needle nose pliers to bend the end of a "slide on" paper clip
to form a loop. Dip the loop into a
beaker of water then tap it against the
side of the beaker to form a water lens inside the loop. The water
lens could
be a convex lens (widest in the middle) or a concave lens (thinnest in the
middle). Examine the
letter "e" with your water lens. Note whether the lens
is a convex lens or concave lens. Dry the loop and
try to make the other
kind of lens.
4.131 Optical bench to study
lenses
See diagram 28.219: Optical bench
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 × 10-7 to 5 × 10-9 metres,
where the X-ray range begins. Ultraviolet radiation
causes sunburn and the
formation of vitamin D in the skin. Ultraviolet rays are strongly germicidal
and may
be produced artificially by mercury vapour lamps for therapeutic
use. The radiation may be detected with
ordinary photographic plates or
films.
4.137 Colours of soap films
Make a strong soap solution as used for blowing soap bubbles. Fill a
flat dish with the solution then dip a
cup into the solution until a soap
film forms across the cup. Hold this in a strong light so that the light
reflects from the film. Note the colours. Tilt the cup to make the film vertical,
and note the changes in the
colour pattern as the film becomes thinner towards
the top. The colours seen in thin films come from the
interference of the
light waves reflected from the front and the back of the film.
4.138 Colours of oil films
1. Add black ink to a flat dish filled with water. Put the dish in a
window where light from the sky is very
bright but not in direct sunlight.
Look into the water so that light from the sky reflects to your eye. While
looking at the water, place a drop of oil on the nearest surface at the
edge of the dish. Note a brilliant
rainbow of colours flashing away from
you towards the opposite edge. Blow on the surface to see a
change in the
colours. Interference of white light results in spectral coloured fringes.
2. Add two drops of clear nail varnish to a bowl of water. Dip black
paper in the water and leave it to
dry. Look at the paper in sunlight from
different angles and see the rainbows form as light is dispersed by
the
layers of nail varnish.
4.139 Colours of transparent
objects, colour filters
See diagram 28.227: Colour filters
Study colour filters. Observe the coloured light that passes through
a transparent object and the colour of
the transparent object. Prepare some
transparent objects with different colours, e.g. coloured glass,
coloured
cellophane. Roll a cylinder with a piece of white paper and fix it vertically
above a piece of white
paper on the table. Put the transparent objects
on the cylinder under sunlight or white light so that light
shines down through
the transparent object. Observe the colour of the paper on the table and
compare it
with the colour of the transparent object. The colours are the
same. Transparent objects absorb some
colours and some colours to pass through
them. They have colour because of the colours they transmit
and that they
absorb all other colours. Water has high transparency. It absorbs some light
in the infrared
and ultraviolet regions of the spectrum but transmits the
visible radiation necessary for photosynthesis.
The colour of a transparent object is a mixture of those wavelengths
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.