School Science Lessons
2018-11-08
Please send comments to: J.Elfick@uq.edu.au

27.0 Light, colour
Table of contents

See: Light, light bulbs, (Commercial)

See: Colour (Commercial)

12.0 Light (Primary)

27.1.0 Colour, spectrum

27.4.0 Electromagnetic waves

27.7.0 Light rays

27.8.0 Photometry

27.1.0 Colour, spectrum
See: Colour (Commercial)
See: Light, light bulbs, (Commercial)
7.0 Colour (Primary)
27.112 Colour wheel, Subtractive colour effects
27.101 Spectrum, electromagnetic spectrum
27.107 Primary colours
27.108 Secondary colours
27.126 Spectral sources, emission spectra of gases
27.106 True colour
27.102 White light and colours of the spectrum
Experiments
27.110 Additive colour effects, complementary colours
27.94 Barber pole, circular polarization (See: 2.) (Experiments)
27.125 Colour caused by absorption, red, green and blue glass
23.8.21 Colour temperature
19.4.13 Colours, food colours, food additives
27.103 Colours of objects
4.138 Colours of oil films
4.140 Colours of opaque objects
4.137 Colours of soap films
27.180 Colours of sunlight, Rainbows, spectrum
4.145 Colours of the sea
4.146 Colours of water
4.144 Colours of the blue sky and the sunset
4.139 Colours of transparent objects, colour filters
4.132 Colours of sunlight, rainbow
27.110 Complementary colours, Additive colour effects,
27.118 Complementary shadow
27.124 Dichromatism
4.114 Dispersion, spectrum with a ray box
4.115 Emission spectrum
27.112.1 Fast colours
4.65 Incandescent lamp, electric light bulb, filament lamp, light globe
4.135 Infrared rays source
27.123 Metal films and dyes
4.143 Mix coloured lights
4.141 Mix coloured pigments, blue and yellow chalk
27.107 Primary colours, rainbow
27.113 Projection of colours
27.180 Rainbows, spectrum
4.142 Rotate colour discs
27.114 See objects through coloured glass
27.115 See flowers through monochromatic light
27.112 Subtractive colour effects
27.116 White froth on a dark-coloured drink, e.g. beer
27.193 Visible spectrum, rainbow

27.7.0 Light rays, Visible light rays
See: Light, light bulbs, (Commercial)
4.65 Light bulb (incandescent filament lamp) (Experiments)
4.120 Light rays through lenses
4.105 Light travels in straight lines, pinhole magnifier
4.103 Low-voltage light source
4.131 Optical bench to study lenses

27.8.0 Photometry, photometers, photoelectric cell, photoelectric effect, light meter
See: Weather station, Light meter, (Commercial)
See: Light, light bulbs, (Commercial)
27.45 Checker board
27.44 Frosted globe surface brightness
27.46 Inverse square law model
27.39 Light bulb brightness, Joly photometer
6.3.1.7 Luminous intensity, candela, cp
27.41 Make a photometer
27.200 Photoelectric effect
27.40 Photometry, photometers, photoelectric cell

27.4.0 Electromagnetic waves, electromagnetic spectrum
4.117 Absorption spectrum
4.132 Colours of sunlight, rainbow
4.114 Dispersion, spectrum with a ray box
4.115 Emission spectrum
27.119 Filtered spectrum
4.65 Incandescent lamp, electric light bulb, filament lamp, light globe
27.123 Metal films and dyes
27.117 Recombining the spectrum
4.134 Spectroscope, diffraction grating
4.136 Ultraviolet light source
27.193 Visible spectrum, rainbow

4.65 Make a model electric light bulb (incandescent filament lamp)
See diagram 28.65: Getting heat and light from electricity
1. Push the ends of two pieces of copper wire through a cork in a small bottle.
Connect the ends of the copper wire inside the bottle with a stand of steel wool.
Connect this model electric lamp model in a circuit with one or more dry cells, or lead cell accumulators, and a switch.
Close the switch until the fine wire filament begins to glow.
At first the heated iron wire produces light but soon he iron combines with the oxygen of the air inside the bottle and burns.

2. Examine a manufactured electric light bulb.
It contains argon but no oxygen.
It has a tungsten carbide wire filament that glows without burning when heated to a high temperature.
The argon restrains the blackening of the inside of the bulb by deposition of tungsten vapour.
Fluorescent lamps containing mercury vapour or neon gas are much more energy-efficient than incandescent lamps.

4.103 Low-voltage light source
See diagram 28.103: Low-voltage light source
Make a compact light source from any small, high intensity electric light bulb that has a short, straight filament, e.g. light bulbs used in
car tail lamps.
Use a small light source to make very sharp shadows with the light bulb filament end on.
Cover the light source with a small drink-can.
Darken the room.
Punch 2 mm diameter holes in the drink-can on all sides.
Blow smoke around the can to make the emerging rays visible.
Make enough holes so that you can see clearly where the light comes from and in what direction it travels.

4.105 Light travels in straight lines, pinhole magnifier
See: Light, light bulbs, (Commercial)
| See diagram 28.105.1: Light travels in straight lines
o | See diagram 28.105.2: Pinhole camera
| See diagram 28.105.3: Shadows
1. Make a pinhole magnifier.
Cut a very small hole through a piece of cardboard with a pin.
Hold the cardboard very close to the eye in good light and look through the hole at some small print.
The print appears larger and clearer because light rays pass through the small hole then spread out.
The small hole functions like a camera shutter keeping out the extra light that would make the image blurred.

2. Look down on a tightly closed fist.
Open the fist very slightly to let the smallest amount of light pass through.
Look at some fine print through the fist.
Move the fist up and down to get the best magnification.

3. Pierce a hole with the pin in the centre of a piece of cardboard.
Hold it 10 cm in front of one eye.
Hold the pin between the card and the eye.
See an upside down image of the pin will be observed.

4. Make a pinhole in a sheet of aluminium foil.
Hold the aluminium foil between a lighted candle and the wall.
See the inverted image of the candle flame on the wall.

5. Hold the hole in the cardboard 3 cm from the eye.
Keep the eyelid almost closed.
See inverted images of the eyelashes.
All objects will cast an upside down image on the retina when the eye is focussed on them.
The brain interprets the upside down image as right side up.

6. Make a pinhole in the middle of one end of a rectangular box, e.g. a shoe box.
Cut a window in the other end of the box and use adhesive tape to attach over it a screen made of greaseproof paper, lunch wrap
paper, baking paper.
Draw the letter T on a piece of thin white paper, or greaseproof paper using a marker pen.
Attach the paper with the T drawn on it to the front of a light source.
In a dark room, direct light from the light source towards the pinhole and, at the other end of the box, look at the image on the screen.
The image of the T is inverted.

4.114 Dispersion, spectrum with a ray box
See diagram 28.114: Dispersion with a triangular prism
Dispersion occurs when light of different wavelengths is spread out by a prism into a spectrum
1. Use a glass prism to produce a spectrum from a parallel beam of light.
Place a card with a narrow slit in front of the lens of a ray box.
se 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.

4.117 Absorption spectrum of sodium
1. When white light passes through a vapour of atoms, they absorb their characteristic wavelengths of light and reduce these
wavelengths in the continuous spectrum emitted to produce a line absorption spectrum.
White light from the sun travels through cooler elements surrounding it that absorb their characteristic wavelengths.
The dark absorption lines in this line absorption spectrum, i.e. solar spectrum, identifies these elements, e.g. Helium.

2. Heat a wire coated in sodium chloride in a Bunsen burner flame and placed in front of a sodium light source.
The sodium vapour from the heated wire appears as a black mist because of its absorption of the characteristic wavelengths of sodium.

4.120 Light rays through lenses
See diagram 28.120: Ray diagrams for lenses
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.

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

4.129.1 Magnifiers, magnifying glass
See: Magnifiers (Commercial)
Magnifying glass, glass lens, magnification × 3.75 mm diameter
Magnifying glass, bifocal, plastic lens, magnification 2 × and 6 ×3.75 mm diameter
Magnifying lens, hand lens, folded magnifier, magnification 10 ×

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
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.

Experiments
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.

4.134 Spectroscope, diffraction grating
See: Diffraction (Commercial)
| See diagram 4.134.1: Spectroscope
| See diagram 4.134.2: Diffraction grating
A diffraction grating is a piece of plastic or glass with many opaque parallel lines rules on it, e.g. 100 lines per mm, 300 lines per mm,
1000 per mm, 13, 500 lines per inch.
When light rays enter the spectroscope, they are separated, according to different wavelengths, into a spectrum or spectra and
produce an interference pattern are sharpened to appear as bright lines of reinforcement (maxima).
Each element has its own characteristic bright lines on its spectrum so the spectroscope is used for chemical analysis.
Spectroscopes are also used in astronomy to determine the elements in the sun and stars, because it can produce separated line images
for light sources with similar wavelengths.
The spectroscope invented by Joseph von Fraunhofer in 1820 used fine parallel wires.

Experiments
1. Make a diffraction grating by drawing evenly-spaced clear black lines on a white card.
Then take a high quality black and white photograph using a camera stand.
Use the negative for a diffraction grating.
However, you can also purchase cheap diffraction gratings as novelty spectacles, called "rainbow glasses".

2. Cut a 2 cm diameter round hole at one end of a cardboard shoe box.
Attach a diffraction grating across the hole on the inside of the box.
Note the direction of the slit on the grating.
In the opposite side of the box, cut a 0.5 cm X 2.5 cm slit opposite the diffraction grating, with the longer side horizontal.
Attach two razor blades to the outside of the slit, almost edge to edge, to form a very narrow vertical slit.
Place a 12 V vertical filament lamp, e.g. a neon lamp or argon lamp, in front of the slit.
Adjust the distance between the two razor blades so that you may see clear linear spectrums when you look through the round hole.
Use the diffraction grating and a sharp source of light to see the order of colours in the spectrum.
ROYGBIV, represents red, orange, yellow, green, blue, indigo and violet.
Note the bright lines in spectra produced by fluorescent mercury lamps and neon signs.

3. Hold a feather near your eye and observe a burning candle far from you.
Adjust the distance of feather from your eye until you see four X-shaped colour bands.
You may also see two blue and two red bands in each of the four bands.

4. Stretch nylon gauze or a woman's fine scarf tightly and observe a burning candle through it.
See colour stripes appearing in the direction of the fibres.
Different weaving and different shapes of small holes will affect different shape of the stripes.
You may see an X-shaped diffraction pattern through some types of nylon gauze.

5. Make a spectrum without a prism.
Set a tray of water in bright sunlight.
Lean a rectangular pocket mirror against an inside edge with the lower part immersed in the water.
Adjust the mirror so that a spectrum appears on the wall.
6. Pass light through a spherical flask of water and view the rainbow on a screen placed between the light and the flask.

4.135 Infrared rays source
See: Thermometers, Infrared thermometers, (Commercial)
| See diagram 28.223: Infrared rays: A Heat lamp, B Visible light, C Iodine solution, D Infrared rays, E Burning black paper.
| See diagram 4.135.1: IR Spectrum pic1 (University of Melbourne)
| See diagram 4.135.2: IR Spectrum pic 2 (University of Melbourne)
Cadmium (II) selenide is transparent to infrared light.
1. Iodine dissolved in alcohol gives a filter transmitting in the IR but absorbing in the visible.
To produce infrared radiation, use a heat lamp for treating muscular ailments.
Fix the infrared lamp on the table so that it shines horizontally on the bulb of a large flask of water.
The flask acts as a lens.
Hold your hand between the lamp and the flask to feel the heat.
Move a piece of black paper on the other side of the flask to find the focal point.
Add iodine solution to the water and shake the flask to make the iodine solution uniform.
Place the flask back at the original position.
Hold a piece of cotton wool soaked in methylated spirit at the focal point.
It starts to burn.
Iodine solution stops visible light but allows the longer infrared wavelengths to pass through.
Infrared radiation is invisible electromagnetic radiation of wavelength between about 0.7 micrometers (0.7 m),
and 1 millimetre (1 mm), i.e. between the limit of the red end of the visible spectrum and the shortest microwaves.
All objects above 0 K, including humans, absorb and radiate infrared radiation.
Infrared radiation is used in medical photography and treatment, in astronomy and in photography in fog.
Infrared radiation can be detected by a Golay cell detector that contains xenon gas.

2. Show that electromagnetic radiation extends beyond the visible into the infrared and its equivalence with heat radiation.
A normal colour spectrum is produced with the aid of the slit and slide projector and the prism.
Rotating the prism will bring different sections of the spectrum into the entrance pupil of the thermopile.
Maximum reading is obtained just passed the red end of the spectrum.
This experiment requires that the infrared filter is removed from the slide projector.
Plastic slides will melt.

3. Set up a slide projector to display a normal spectrum on the screen.
Remove the IR filter and place a 2-3 mm slit in the slide carriage.
Focus a digital movie camera on the image and compare the images in normal mode and night vision mode.
The CCD elements are sensitive to the infra red and normally an IR filter is used to block the IR.
In night vision mode this filter is swung out of the way, allowing the infra red to be displayed.

4.136 Ultraviolet light source
See diagram 28.224: Ultraviolet light source
1. Attach two lamp holders to insulating material and fasten it to the bottom of a cardboard carton with the top removed.
Fix two argon lamps into the lamp holders and connect the lamps in parallel without leaving any bare wire exposed.
Cut a notch in the side or end of the box for the electrical lead cord.
Invert the box cut a peephole to allow viewing without direct eye exposure to the ultraviolet light.
Ultraviolet light may cause serious damage to the eyes.
However, you can observe different objects in "black light" by placing the cardboard box over the objects, turning on the switch
to the power source and observing the objects through the peep hole.
Objects that glow under ultraviolet light include clothing dyed with fluorescent dyes, e.g. socks and ties, soap
powders containing an "optical brightener", e.g. "Bluo", and white clothes washed in these powders,
fluorescent paints and lacquers, fluorescent chalk, some minerals, e.g. willemite, fluorites, opals and sphalerites.

2. Use an argon lamp as an ultraviolet light source to display fluorescence.
Mount an argon lamp in a light proof box and cut a peephole in the box for viewing.
Be careful! Avoid direct eye exposure to the ultraviolet light, which may damage the eyes.
To note different objects in black light, put the box over the objects and turn on the argon lamp.
Clothes may contain fluorescent dyes, e.g. bright socks.
Ultraviolet rays in ordinary sunlight cause fluorescent dye to glow.
Soap powders may contain a brightener.
White clothes washed in these powders fluoresce in the ultraviolet radiation from the sun or from an argon light bulb.
Fluorescent paints, lacquers and chalk are also available.
Some minerals fluoresce in ultraviolet light, e.g. ilmenite, opal, sphalerite and some fluorites.

3. Collect objects that glow under ultraviolet light.
Ultraviolet light is used for bank note testing, in hospitals and in fluorescent watches.
Ultraviolet radiation is light rays invisible to the human eye, of wavelengths from about 4 × 10-7 to 5 × 10-9 metres, where the X-ray
range begins.
Ultraviolet radiation causes sunburn and the formation of vitamin D in the skin.
Ultraviolet rays are strongly germicidal and may be produced artificially by mercury vapour lamps for therapeutic use.
The radiation may be detected with ordinary photographic plates or films.

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

4.138 Colours of oil films
1. Add black ink to a flat dish filled with water.
Put the dish in a window where light from the sky is very bright but not in direct sunlight.
Look into the water so that light from the sky reflects to your eye.
While looking at the water, place a drop of oil on the nearest surface at the edge of the dish.
Note a brilliant rainbow of colours flashing away from you towards the opposite edge.
Blow on the surface to see a change in the colours.
Interference of white light results in spectral coloured fringes.

2. Add two drops of clear nail varnish to a bowl of water.
Dip black paper in the water and leave it to dry.
Look at the paper in sunlight from different angles and see the rainbows form as light is dispersed by the layers of nail varnish.

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

4.140 Colours of opaque objects
1. Note the colour of a piece of red cloth in white light or sunlight.
In a dark room, note the colour of the same piece of red cloth in red, blue, green, and yellow.
The red cloth appears black unless placed in light of the same colour or in white light or sunlight.
Opaque objects have colour because of the light they reflect.
In white light or sunlight they absorb the other colours of the spectrum.
Repeat the experiment with a piece of white cloth.
White objects may reflect any colour.
Repeat the experiment with a piece of black cloth.
Black objects absorb all colours and do not reflect any colour.
Repeat the experiment with coloured illustrations from a magazine.
In white light or sunlight, remember the colour of each part, e.g. red flowers and green leaves, then compare its colour under the coloured light.

2. Note the colour of dry sand.
Add water to the sand and note any change of colour.
Dry sand is composed of pieces of quartz that reflect light in all directions so that the sand appears almost white.
When sand is wet, the layer of water on each quartz grain reflects back some light at the air water surface, so the sand appears darker.

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
Light, Newton's colour disk, (Commercial)
See diagram 28.230: Rotate colour discs
1. Mix coloured lights by using water colours painted on discs of cardboard.
Paint a yellow "egg yolk" on one side of a 10 cm disc, and a blue "yolk" on the other side.
Suspend the disc between loops of string.
Twist the loops then pull outwards to make the disc spin.
The resulting colour is nearly white.

2. Paint radial segments alternately red and green.
Note the resulting mixture of red and green lights reflected to the eye by spinning the disc on a string.
3. Divide a white disc into seven segments.
Paint each segment with one of the seven colours of the visible spectrum, (violet, indigo, blue, green, yellow, orange, red).
Spin the disc rapidly, e.g. attached to an electric motor.
The disc appears nearly white, depending on the purity of the colours.
This disc is called Newton's disc or Newton's colour wheel.

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

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

Experiments
1. Observe ripples of water passing through upright reeds and note that shorter wavelength ripple are scattered more by passing
through the reeds than longer wavelength ripples.

2. Shine a narrow beam of light through a fish tank or a large beaker filled with water.
Add drops of milk or powdered milk or acidified sodium thiosulfate solution while stirring until you can see the beam shining through
the water.
Look at the beam both from the side and from the end, where the beam shines out of the container.
Viewed from the side, the beam appears blue.
Viewed parallel to the direction of the beam, the beam appears orange-red or yellow.
See the colour of the beam change from blue-white to orange-yellow along the length of the beam.
Let the light project onto a white card at the end of the tank.
The beam spreads so it is not so narrow as at the source of light.
Particles in the milk scatter the light and so allow you can see the beam from the side.
Blue light is scattered much more than orange light or red light, so we see more blue light from the side.
Orange light and red light are scattered less so we see them at the end.
The shorter wavelength blue light has a greater refractive index so it bends more than longer wavelength red light with a smaller
refractive index.
Similarly, atmospheric gases smaller than one wavelength scatter blue light, so the sky appears blue.
This phenomenon is called Rayleigh scattering.
The sun is white hot but it appears orange-red because the white light from it has lost some blue light.
When the sun is on the horizon, its light takes a longer path through the atmosphere to your eyes than when directly overhead.
So at sunset most of the blue light is lost by scattering leaving the orange-red light, i.e. white light minus blue light.
Only the longer wavelengths reach the eyes.
If there were no scattering, and all the light from the sun travelled straight to the earth, if not looking at the sun,
the sky would look dark as it does at night.
Large particles, e.g. dust, smoke, and pollen, scatter light without breaking white light into component colours.
This is called Mie scattering.
It is the cause of the whiteness of clouds, mist, milk, latex paint and the white glare around the sun and moon during a mist.
The sun has the same colour as a black body at 5780 K.

3. Place a lens from Polaroid sunglasses between the light source and the fish tank.
Hold the lens vertically and turn it while another person observes the beam from above and another person observes the beam from
the side.
When the person above observes a bright beam, the person at the side observes a dim beam, and vice versa.
This is the same effect when look through two parallel sun glass lenses and you turn one of the lenses.
At a certain position no light, or very little light, passes through both lenses.
So the scattering in the fish tank polarizes the light.
Light emitted by the sun, by a lamp in the classroom, or by a candle flame is unpolarized light.
Electromagnetic light waves from the sun or an electric lamp come from electric charges vibrating in many directions perpendicular to
the direction of the light beam.
Sunglasses include a Polaroid material that absorbs light vibrating horizontally and so reduces glare.
So the light reaching your eyes is polarized light.

4.145 Colours of the sea
The sea appears blue because it absorbs all of the wavelengths of sunlight except the short blue wavelength.
The oxygen content of water molecules absorbs the red end of the spectrum.
Blue light is scattered in water in all directions to cause the blue oceans.
Similarly at the North and South polar regions the ice and icebergs appear blue.
The blue colour changes if the sea contains phytoplankton, suspended sediments, and dissolved organic chemicals
as in the seas in the temperate regions.

4.146 Colours of water
1. Observe from above the water in the deeper end an indoor swimming pool with white ties and illuminated with white light.
It appears blue because the red component of the light reflecting from the bottom of the pool is absorbed.
It looks less blue at the shallow end of the pool.
Thus, as white light travels through water, the red / orange / yellow components of light get absorbed by the water and cause the water
molecules to vibrate, while high frequency (blue) photons continue to travel through the water.
The net result is a slight increase in the water temperature (molecular vibrations translate directly into temperature), and the white light
turns blue as it travels deep through the water.
The presence of dissolves salts, sediments and algae may affect the colour of water.

2. Poke a hole in the snow with a 1 m long stick or ski pole.
The hole looks darker and blue.
Ice containing many small air bubbles appears white.

6.3.1.7 Luminous intensity, candela, cp
A candela is the luminous intensity in a given direction, of a light source that emits monochromatic radiation of frequency 540 × 1012
hertz and that has a radiant intensity in that direction of (1 / 683) watt per steradian.
It is the unit of luminous intensity equal to 1 / 60 of the luminous intensity per square centimetre of the surface of a black body at the
temperature of solidification of platinum.
The previous unit was the candlepower, about 0.98 of a candela, that was defined in various ways, including the light from a standard
whale oil candle.
However, people liked to continue to use the term candlepower, so nowadays 1 candlepower = 1 candela.
The zirconium wire in a camera flash cube ignites to release a 2 000 modern candlepower burst of light for about 30 millionths of a
second.

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

27.39 Light bulb brightness, Joly photometer, wax block photometer
See diagram 28.2.4: Make a photometer
Electric energy can be transferred not only into light energy but also heat when light bulb works.
So its efficiency can be expressed as the ratio of luminous intensity to consumed electric power.
Light intensity at distance s from a light source varies inversely with the distance squared.
It can be measured with a light meter or a photometer.
If the light meter is calibrated to the size of camera length apertures, it is called an exposure meter.
1. Using a Joly photometer (wax block photometer)
It consists of two equal paraffin wax blocks separated by a thin opaque sheet.
You can adjust the positions of two light sources to be compared until the two wax blocks appear equally bright.
Also known as The Joly photometer is made from two identical blocks of paraffin wax, B1 and B4, about 5 mm thick, separated by a
sheet of aluminium foil.
Luminous sources of light, intensity I1 and I4, are placed each side of the blocks at distance S1 and S4 from the aluminium sheet, so
that B1 receives illumination only from S1 and B4 receives illumination only from S4.
By viewing from the side, i.e. in the plane of the aluminium sheet, the intensity of the diffused light from the paraffin blocks can
be compared.
If the photometer is moved between two light sources so that the light intensity seen in each block is the same, then
I1 / S14 = I4 / S44.

Photometry, photometers, photoelectric cell
Brightness and efficiency of light bulbs, photometer, luminance and illuminance, incandescent lamp, photoelectric cell, intensity of light,
inverse square law, photoelectric exposure meter
Experiments
1. Make a paraffin block photometer, Joly diffusion photometer, using two large paraffin blocks with tin foil sandwiched in between
make a sensitive photometer.
Use with lamps on either side.
Two paraffin blocks separated by an aluminium sheet are moved between two light sources until they appear equally bright.
2. Make a grease spot photometer, Bunsen grease spot photometer, using a piece of paper with a grease spot is moved between two
light sources until the spot disappears.
A grease spot disappears when illuminated equally from both sides.
3. Make a Rumford shadow photometer using light sources moved until their shadows of the same object are of equal intensity.
Two light sources are moved so the shadow cast by a vertical rod is of the same intensity.

27.41 Make a photometer
See diagram 28.2.4: Make a photometer
Use a rectangular cardboard box, e.g. a school chalk box.
Cut four identical rectangular windows in the sides of the box.
Make two paraffin blocks each 5 mm thick and half the area of the window so that the two blocks can just fit side by side in the window.
Make sure that the upper and lower surfaces of the paraffin blocks are smooth.
Cut a piece of flat aluminium foil the same size and shape as the paraffin blocks.
Fit it between the blocks and fit the blocks and foil into the window.
Fix two globes in lamp holders each side of the box.
One globe of known light intensity, e.g. 40 watt frosted bulb, luminous intensity about 32 candelas.
The luminous intensity of the other globe is unknown.
Darken the room and turn on the power for the two globes.
Slide the photometer to a position where the two sides of the paraffin blocks are equally bright.
Record the distances from the
aluminium foil sheet to each globe.
If I1 = known intensity, e.g. 32 candelas and I4= unknown intensity then as I1 / S14 = I4 / S44, I4 = (32 / S14) / S44.

27.44 Frosted globe surface brightness
The surface brightness of a 40 W bulb is compared to a frosted globe placed over it.

27.45 Checker board
Use a point source to superimpose shadows of a rectangle and a 3h × 3w checkerboard rectangle.

27.46 Inverse square law model
| See diagram 27.4.1.6a: Shadow of 4 squares, (University of Melbourne)
| See diagram 27.4.1.6b: Shadow of 16 squares, (University of Melbourne)
1. Set up a light source to cast a shadow of a square onto a reference grid.
Adjust the distance of the source to the grid to 20 cm.
Move the object 10 cm from the source so that the shadow covers exactly 4 squares on the grid.
Double the distance between the source and grid.
The shadow cover 16 squares.

See diagram: 27.46: Inverse square law model
2. Place pointed end next to a light source.
As the light crosses the first plane, we see a square area that is equal to one unit of light.
At the next level we notice that the same amount of light now covers four times the area.
Therefore, it is 1/4 as bright per unit area as the first level.
At the bottom level, we notice it covers nine times the area or it is 1/9 as bright.

3. Use a Weston type foot-candle meter to measure the inverse square law.
4. To see the surface brightness of a lens, place the eye at the image point of a lens focussed on a dim lamp.
5. To see reflected surface brightness, with a bright spot at the object point of a concave mirror and the eye at the image point the
whole mirror seems to have the same surface brightness as the spot.
6. Focus a beam of light intermittently on a vane of the quartz fibre radiometer at the frequency of oscillation.

27.83 Polarization, Polaroid
Polarized light has vibrations restricted to a particular direction.
Polarized light has the changing electric field component in one plane.
Polarizers, e.g. as in Polaroid sun glasses, allow only one plane of changing electric field to pass through them.
Polaroid consists of thin sheets of material that produces a high degree of polarization of light passing through it, e.g. Polaroid lenses in
sunglasses and polaroid lenses for some cameras.
1. Strain polarization interference
See diagram 27.6.1.0: Strain polarization, (University of Melbourne)
Place perspex models and strained glass caused by rapid cooling on the overhead projector with the polaroid sheets in the crossed
position.
Strain in the glass or induced in the perspex causes the object to become anisotropic and birefringent to form visible stress lines due to
interference colours.

2. Polaroid on the overhead
See diagram 27.83.2: Polaroid on the overhead
1. Use two sheets of Polaroid and a pair of Polaroid sunglasses.
Rotate one on the Polaroids or the sunglasses.
Examine polarization with two sheets of Polaroid and a pair of sunglasses on an overhead projector.
2. Two Polaroid sheets are partially overlapped while aligned and at 90o.
3. A beam from an arc lamp is directed through two Polaroid sheets.
.

3. Three Polaroid sheets, circular polarization
See diagram 27.83.3: Circular polarization
1. Use three sheets of Polaroid with an overhead projector.
Insert a third sheet between crossed Polaroids.
Show polarization with two sheets of Polaroid and a pair of sunglasses on an overhead projector.

4. Change colours of corn syrup
See diagram 27.83.4: Corn syrup
Place the corn syrup between the disks.
Place the light source behind the rear polarized disk and turn the lamp on.
Rotate the front disk and watch the syrup change colours.
3.16 Corn syrup mountant, microscopy stain.

5. Polarization mechanical model, pendulum, Hang a pendulum from a long strut restrained by slack cords.
Circular motion of the pendulum will be damped into a line by the motion of the strut.
6. Cut squares of Polaroid so the axes are at 45 degrees.
Now turning one upside down causes cancellation.
7. Tilt the windowpane for reflection polarization.
Reflect plane polarized light off a window pane and vary the angle of incidence through Brewster's angle.
8. Stack glass plates at 57o to transmit and reflect light that is cross polarized.
9. For circular polarization, insert a tube of sugar solution between crossed Polaroids.
Compare the rotation of plane-polarized light in tanks containing sugar solution, turpentine and water.

27.88 Brewster's angle, reflection polarization
See diagram 27.88: Reflection polarization
Reflection from a dielectric surface, glare, reflected light is partially polarized at right angles to the plane of incidence.
Brewster's angle: reflected light is completely polarized when the reflected ray is perpendicular to the refracted ray.
The tangent of Brewster's angle equals the index of refraction.
Experiments
1. Light is reflected from a sheet of black glass onto the wall.
Rotate a sheet of Polaroid in the path of the reflected light.
Set the light at Brewster's Angle.
Vary the incident angle by rotating the light.
A metal triangle can be used to set the incident light back to Brewster's Angle.
2. Rotate a Polaroid filter in a beam that reflects at Brewster's angle off a glass onto a screen.
A beam of white light is reflected off a sheet of black glass at Brewster's angle onto the wall.
Use a Polaroid to test Brewster's angle.
3. Black glass, reflection polarization
See 2.4: Canada balsam
Eliminate the reflection off the second surface of a glass plate with a Canada balsam and lampblack suspension on the back side.

27.94 Barber pole, circular polarization
| See diagram 27.6.3.5a: Barber pole equipment
| See diagram 27.6.3.5: Barber pole tube (not rotating) (University of Melbourne)
1. Rotate a beam of polarized light when directed up a vertical tube filled with sugar solution.
Examine a beam of polarized light up through a tube with a sugar solution and scattering centres.
The beam rotates and colours are separated.
2. Show that the rotation of the plane of polarization in a sugar solution, i.e. the optical activity, depends on wavelength.
Reflect a parallel beam of light into a large beaker containing a sugar solution of 3 parts sugar in 4 parts of water by weight.
Polarize the incoming light with the polaroid sheet mounted on the spindle of the motor M.
As the light passes through the sugar solution, the sugar molecules rotate the plane of polarization depending on the wavelength so a
spiral of rainbow colours is seen.
As the polarizer rotates the spiral rotates.
Use another polarizer above the beaker to receive the transmitted light to form a single broad beam of light on the ceiling with each
colour of the spectrum as the polarizer is rotated.
3. Illuminate a tube of corn syrup from the bottom.
Insert and rotate a Polaroid filter between the light and tube to see a "barbershop" sugar tube.
4. Insert a partially filled glass container of wax into the core of a solenoid between crossed Polaroids to see Faraday rotation.

27.101 Spectrum, electromagnetic spectrum
See: Colour (Commercial)
See diagram 27.101: Colour wheel
The term spectrum may refer to:
1. The electromagnetic spectrum, i.e. the range of electromagnetic radiation from the longest radio
waves to the shortest gamma rays.
2. The coloured produced when a beam of light is split by a prism or a diffraction grating.
3. A band of bright and dark lines, characteristic to a particular light source, the bands corresponding to the frequencies emitted or
absorbed by the light source.
4. A characteristic pattern of absorption and emission of electromagnetic radiation.

27.102 White light and colours of the spectrum
See: Colour (Commercial)
See diagram 28.133: Electromagnetic spectrum
White light consists of all the colours of the spectrum.
Colour is quality or wavelength of light emitted or reflected from an object.
Visible white light consists of electromagnetic radiation of various wavelengths, and if a beam is refracted through a prism, it can
spread into a spectrum, in which the various colours correspond to different wavelengths.
White light is compounded of all the wavelengths in the proportion in which they would occur in sunlight.
The colours are red, orange, yellow, green, blue, indigo, and violet.
So white light could be defined as the light emitted from a perfect radiator at a temperature of 6 000 degrees absolute, the temperature
of the radiating surface of the sun.
However, sunlight already lacks in many wavelengths before it leaves the sun's atmosphere.
Also, absorption of wavelengths in the earth's atmosphere is much greater for short wavelengths, violet to blue colours, than for longer
wavelengths, green to yellow and orange colours, than for long wavelengths, red colour.
Sunlight may be rich in long wavelengths, red, because of the diffraction or scattering effects of dust particles in the atmosphere when the
sun is near the horizon.
The uninterrupted light from a very hot radiator, wavelength 10 000 A or less, may be called white light.

27.103 Colours of object
See: Colour (Commercial)
The colour that appears when white light illuminates an object is called the colour of the object.
It depends on the selected absorbing and selected reflection of light by the object.
When you illuminate a surface, some parts of the white light are absorbed, depending on the molecular structure of the material and the
dyes applied to it.
A surface that looks red absorbs light from the blue end of the spectrum, but reflects light from the red, long wave end.
Colours vary in brightness, hue, and saturation, the extent to which they are mixed with white.
As the red, green and blue light mix according to a ratio of their brightness you can obtain various colours of light.
They are called the three basic colours of light.
The mixture of equal amounts of three basic colours makes no colour light, white light.
Three conditions for colour
1. The colour must be in the source
2. The object must reflect or transmit the colour.
3. The detector must be sensitive to the colour

27.105 Additive colour
In an additive colour effect all the wavelengths present in both, or all, the colours are present in the resulting colour.
Additive colour effects can be produced physically by mixing coloured lights or psychologically using a rotating disc with colours on it
in sectors.
Owing to the persistence of vision the eye sees all the colours on the colour disc combined.
So the combination of blue and red lights, or blue and red sectors on the colour disc, gives purple.

27.106 True colour
See: Colour (Commercial)
An object only shows its true colour when the incident light contains, all the wavelengths capable of being reflected by the body, and
contains them in the same proportion as they occur in white light.
Otherwise the colour seen depends on the wavelengths in the light that are reflected by the body, a sort of subtractive effect.
For example, in yellow light a true blue body appears black but if the blue body it reflects some blue, green, and yellow, it will appear
yellow.
If the light contains a larger proportion of one colour than does white light, then the body reflects larger proportion of this colour than it
would in white light.
An impure green body viewed in a yellow light will have the yellow in it increased.

27.107 Primary colours, rainbow
See: Colour (Commercial)
"RGB Strobe Ring", to separate white light into the 3 primary colours, (toy product).

Primary colours are any of the colours from a mixture of which all other colours can be produced.
Sometimes the seven colours of the rainbow are called primary colours.
Two types of primary colours, for lights and paints
1. For additive combination of colours, red, green, and blue (violet), colours are primary colours because they can produce all other
colours, hues, when a selection of these colours is projected on the same screen.
When red + green + blue light is projected on the same screen, the reflected light is white, so white light is seen.
Overlapping primary colour lights produce secondary colours.
2. For subtractive combination of colours, red, yellow and blue colours are the primary colours of pigments, paints or dyes because
they can produce all other colours when mixed.
When red + green + blue pigments are mixed, there is no reflected light so black should be seen, but most pigments are not pure so
usually purple is seen.
The colours red, yellow and blue (or violet) are called "primary colours" because they cannot be made by mixing other colours.

27.108 Secondary colours
See: Colour (Commercial)
See diagram 27.1.08: Mixing primary colours
Secondary colours can be formed by shining red, green and blue light on a white screen, so they overlap partially.
Where red primary colour overlaps blue primary colour --> magenta secondary colour.
Where blue primary colour overlaps green primary colour --> cyan, (peacock blue, blue-green), secondary colour.
Where green primary colour overlaps red primary colour --> yellow secondary colour.
Where red primary colour overlaps blue primary colour overlaps green primary colour --> white.
If magenta secondary colour overlaps cyan secondary colour overlaps yellow secondary colour --> white.

27.110 Additive colour effects, complementary colours
A primary colour with its opposite secondary colour are called complimentary colours.
Primary colour + opposite secondary colour --> white
Red + cyan --> white
Blue + yellow -- > white
Green + magenta --> white
(Note: nm = nanometre = 10 Angstrom units = 10-9 m.
So Non-SI unit "angstrom" = 0.1 nanometres.)
These colours can be combined to give the visual effect of white light.
For example, red light, 6562 Angstrom wavelength, and green blue, 4921 Angstrom wavelength, are complementary, so the resulting
additive effect is white light.
Additive effects occur when the progressive waves comprising the different colours are added.
Project these two colours from separate sources onto a white screen.
The reflected light is an additive effect and so appears white.

27.112 Subtractive colour effects, colour wheel
See: Light, Newton's colour disk, (Commercial
Pigment colour is created when a pigment absorb certain light wavelengths and reflects others.
For example, a blue shirt absorbs all wavelengths except blue, which is reflected.
The colour wheel based on the three primary colours (red, yellow and blue), was developed in 1666 by Sir Isaac Newton.
Primary pigment colours (red, yellow and blue), are the primary colours.
All other colours are derived from these three hues.
Secondary pigment colours (green, orange and purple), are created by mixing the primary colours.
Tertiary colours (yellow-orange, red-orange, red-purple, blue-purple, blue-green and yellow-green), are the colours created by mixing
the secondary colours.
In a subtractive colour effect, only those wavelengths common to both colours are present in the resulting colour.
Subtractive colour effects are obtained by mixing pigments or superposing sheets of coloured transparent material.
If you mix blue and yellow pigments, the blue and yellow are probably not pure.
The blue absorbs practically all the red, orange and yellow and reflects a large proportion of green, and most of the blue, so it appears
blue.
The yellow pigment absorbs the violet and blue, and reflects most of the green, yellow and orange and some red.
The only colour reflected by both pigments is the green, and so the colour of the mixed pigments is green.
All the other colours are absorbed by one of the pigments.
These effects are produced when different substances subtract groups of wavelengths from the original light, as in the mixing of pigments.

Experiments
1. Mix red and yellow pigments.
The mixture appears orange, a mutual colour.
2. The subtractive effect for pure blue and pure red is black.
Mixing pigments of blue and red gives black when the colours are saturated, i.e. no white light mixed with colour.
In practice, the resulting colour is generally purple.

27.112.1 Fast colours
It refers to the colours of dyes that do not readily wash out in clothes washing water due to their chemistry.

27.113 Projection of colours
1. Use four lanterns to project on a white screen slides coloured red, yellow, green, and blue.
The reflected light is an additive effect and so the screen appears white.
2. Mount the four slides coloured red, yellow, green, and blue on the one projector so that light from the projector passes successively
through the four slides.
The red slide transmits the wavelengths that constitute red, but absorbs the other wavelengths.
Similarly the other slides do the same.
The light transmitted by one slide will be absorbed by another slide. There is no reflected light due to the subtractive effect so the screen
appears dark.

27.114 See objects through coloured glass
See: Colour (Commercial)
Examine a red, white and blue flag through red glass and then a blue glass.
When examined through red glass, the red regions appear deep red, the white regions appear red, and the blue regions appear black.
When examined through blue glass, the blue regions appear deep blue, the white regions appear blue, and the red regions appear black.

27.115 See flowers through monochromatic light
Monochromatic light has waves of a single frequency, so single colour.
Examine a bowl of flowers of various colours is illuminated by monochromatic yellow light.
The flowers appear various shades of yellow to black, depending on the amount of yellow light reflected.

27.116 White froth on a dark-coloured drink, e.g. beer
Examine the white froth on a dark drink, e.g. beer.
When light passes through a transparent coloured body the amount of absorption and so the depth of colour seen depends on the
thickness of the body.
The film of liquid surrounding the bubbles of air in the froth is very thin and so the absorption of light passing through it is negligible.
The white froth is seen by light reflected from the bubbles.

27.117 Recombining the spectrum
Recombine the spectrum after passing through a prism to get white light or remove a colour and get the complement.
Obtain a spectrum with a prism, reflect out a colour with a small thin mirror and recombine the light with a lens.

27.118 Complementary shadow
Shadows of red and white lights illuminating the same object from different angles appear to produce green light.

27.119 Filtered spectrum
Part of a beam of white light is projected through a prism.
When a filter is inserted in the beam the spectrum and transmitted light are compared.

27.122 Absorption spectrum of chlorophyll
Examine the absorption spectrum of chlorophyll obtained by macerating leaves in methyl alcohol.

27.123 Metal films and dyes
A thin film of gold transmits green but looks red yellow by reflection.
Dyes also transmit and reflect different colours.

27.124 Dichromatism
1. Having two colours, e.g. bromophenol blue, resazurin, pumpkin seed oil.
Observe a thin layer of pumpkin seed oil.
It is green.
Observe a thick lay of pumpkin seed oil.
It is red.
Dichromatism can be explained by the Beer-Lambert law of absorption of light by substances.
2. Green cellophane transmits more red light than green.
Stack lots of sheets and the colour of transmitted light changes from green to red.

27.125 Colour caused by absorption, red, green and blue glass
Light from a projection lantern reflected off red, green and blue glass to the ceiling is the same but the transmitted light is coloured by
absorption.

27.126 Spectral sources, emission spectra of gases
See: Diffraction (Commercial)
Spectral sources include: hydrogen, neon, sodium, carbon dioxide, xenon.
The emission spectra may be viewed using following:
1. Direct observation to show their overall colour
2. Use of direct vision spectroscopes.
Place a large diffraction grating in front of each source.
View the image on a spectrometer.
The spectrometer is aligned in the normal way to produce a spectrum at the position of minimum deviation.
The telescope is swung out of position and replaced with the video camera focussed on infinity.
The colour camera gives a fairly faithful rendition of the spectral lines.

27.127 Dispersion colour and deviation spectrometry, deviation through a prism
White light consists of all the colours of the spectrum.
Dispersion is the splitting of white light into the colours of the spectrum (violet, indigo, blue, green, yellow, orange, red).
Refractive index violet light > refractive index red light so violet light refracts more than red light.
Single colour monochromatic light does not disperse.

27.180 Rainbows, spectrum
See diagram 28.220: Colours of sunlight
The rainbow consists of nearly circular arcs of colour with a common centre.
When you see a rainbow the sun is behind you and the common centre is in the direction to the sun.
Rain is falling in the direction of the rainbow.
When you see a rainbow, note the time and angle of elevation of the sun.
The rainbow is part of a circle with its centre below the horizon.
When the sun is higher than 42o the rainbow is completely below the horizon.
So a rainbow can be seen in the morning or afternoon but not at midday.
Rainbows are usually seen about individual cumulus or cumulonimbus clouds that have gaps between them to allow sunlight to fall onto
raindrops.
The sunlight enters the raindrops and reflect off the inside of the far surfaces to return towards the sun.
Different wavelengths reflect at different angles to split the spectrum.
The light from a rainbow comes towards the observer in the same way that sunlight reflections on the sea surface come towards the
observer.
The sky within the rainbow appears brighter than outside it.
A secondary dimmer rainbow with reverse order of colours may appear within the primary rainbow.
A dark region between the primary and secondary rainbows is called Alexander's dark band.
Rainbows are seen in fogs, fog bows, when sunlight from behind the observer passes through a break in the fog.
Also a rainbow may be seen from an aircraft window when looking down on the shadow of the aircraft on cloud below.
A corona may be observed around the moon consisting of a central white disc wider than the moon with a faint spectrum ring of colour
around it.

Experiments
1. Use a shallow dish of water to form a spectrum on the wall or on a screen.
The light from the sun has to first pass through the water then be reflected back on the wall by the mirror.
This experiment needs very fine adjustment to the angle of the mirror.
Also the spectrum forms only when the water is still so careful adjustment and patience is needed!
2. Make a spectrum with a fine spray garden hose.
Most children will have seen the rainbow produced from the fine spray of the garden hose in the sunlight.
3. Time of appearance of a rainbow
4. Artificial rainbow
Form a vertical circle rainbow by placing a tube of water between a prism and screen.
Use a single sphere with the back surface coated with a reflecting material to show both primary and secondary bows with increased
intensity.
5. Rainbow droplets
Small droplets formed by spraying an atomizer on a soot-covered glass plate glisten like coloured jewels when viewed at different
degrees.
6. Arc lamp
An arc lamp directed at a sphere of water forms a rainbow on a screen rainbow.

27.185 Wave front models
Wire models show spherical and elliptical wave fronts in crystals.

27.190 Sunset with polarizer
Use a sheet of Polaroid to check the polarization of scattering from a beam of light passing through a tank of water with scattering
particles.
Rotate a Polaroid in the incoming beam or at the top and side of the tank in the sunset demonstration.

27.193 Visible spectrum, rainbow
See diagram 28.133: Electromagnetic spectrum
Sunlight through prism, recombining spectrum, rainbow, spectroscope, electromagnetic spectrum
The spectrum is the arrangement of frequencies or wavelengths when electromagnetic radiation are separated into their constituent parts.
Visible light is part of the electromagnetic spectrum and most sources emit waves over a range of wavelengths that can be broken up
or "dispersed".
White light can be separated into the seven colours of the spectrum: red, orange, yellow, green, blue, indigo, and violet, also called the
fundamental colours.
Light scattered from small molecules is polarized at right angles to the direction of propagation of the original beam.

27.200 Photoelectric effect
The photoelectric effect is the loss of electrons from a metal surface due to electromagnetic radiation hitting the surface.
A "particle" of light or photon has a specific amount of energy called a quantum of energy.
Photons of light of higher energy have higher frequency, v, and shorter wavelength, X.
Photocell (vacuum type) threshold frequency is the minimum frequency of radiation that will just produce the photoelectric effect and is
different for different metals, e.g. Magnesium has a lower threshold frequency than copper so it is a more sensitive.
Photoelectric cells can produce electricity.
A photon booked a flight at an air terminal and the clerk asked: "Do you have any luggage?"
The photon replied: "No, I am travelling light!"