School Science Lessons
27. Light, wave and particle behaviour, colour, diffraction, dispersion, spectrum, photoelectric, polarization, rainbows, IR, UV
2012-05-04b SP
Please send comments to: J.Elfick@uq.edu.au
Table of contents
27.0.0 Light
27.0.0 Light, wave or particle nature of light, speed of light, optics
16.0 Light experiments (Primary)
4.132 Colours
27.3.0 Diffraction
27.1.50 Dispersion
27.1.0 Electromagnetic radiation
27.1.0a Electromagnetic radiation, Examine X-ray photographs
27.1.0b Radiation hazards
27.2.1 Infrared radiation (IR)
27.5.0 Interference
7.5.0 Laser safety
See pdf: Light modulation, flashlight connected to iPod, MP3, CD Player
28.1.0 Light rays and shadows
28.0.0 Light rays, optics
28.1.1 Light rays, speed of light
28.4.0 Light sources, producing light
14.3.0 Light stick, (chemical light stick), chemiluminescence, bioluminescence
4.105 Light travels in straight lines, pinhole magnifier
36.14.2 Light year, parsec
.5.8.0 Motor vehicle lighting systems and equipment
4.131 Optical bench to study lenses
28.11.0 Optical devices (low-cost)
27.4.0 Photometry, photometers, photoelectric cell, photoelectric effect
27.6.0 Polarization
27.4.2 Radiation pressure, "light pressure"
28.3.0 Reflection of light at curved surfaces, curved mirrors
28.2.0 Reflection of light at flat surfaces, plane mirrors
28.5.0 Refraction of light
27.1.80 Rainbows
27.2.2 Ultraviolet radiation (UV)
4.132 Colours
See pdf: Light and colour
4.132 Colours experiments, UNESCO
4.117 Absorption spectrum
27.1.05 Additive colour
35.5 Colour, (Geology)
27.1.19 Colour caused by absorption
27.1.03 Colours of objects
4.138 Colours of oil films
4.140 Colours of opaque objects
4.137 Colours of soap films
4.132 Colours of sunlight, rainbow
4.144 Colours of the blue sky and the sunset
4.145 Colours of the sea
4.139 Colours of transparent objects, colour filters
27.1.1 Complementary colours
27.1.12 Complementary shadow
27.1.18 Dichromatism
3.9 Different colours (Primary)
27.1.09 Fast colours
27.2.1 Infrared radiation (IR)
27.1.14 Liquid cell absorption
27.1.17 Metal films and dyes
4.143 Mix coloured lights
4.141 Mix coloured pigments, blue and yellow chalk
27.1.07 Primary colours
27.1.3 Projection of colours
3.10 Rainbow colours (Primary)
27.1.11 Recombining the spectrum
4.142 Rotate colour discs
27.1.08 Secondary colours
27.1.4 See objects through coloured glass
27.1.5 See flowers through monochromatic light
10.2.0 Separate with chromatography
37.34 Solar ultraviolet radiation and skin cancer
3.11 Spin a colour disk (Primary)
27.1.2 Subtractive colour effects
23.8.8 Surface colour and the heat absorbed
27.1.01 Three conditions for colour
27.1.06 True colour
27.1.6 White froth on a dark-coloured drink
27.1.02 White light
27.3.0 Diffraction
4.134 Diffraction grating, spectroscope
4.92 Diffraction in a ripple tank
4.119 Diffraction of light
27.5.2.5 Gratings in air and water
27.5.2.7 Random multiple gratings
27.3.1 Single slit and laser
27.5.2.8 Speckle spots and random diffraction
27.3.2 White light diffraction
27.1.02 White light
27.1.50 Dispersion
27.1.15 Band absorption spectrum
27.1.50 Dispersion colour and deviation spectrometry, deviation through a prism
27.1.51 Dispersion curve of a prism
27.1.53 Dispersion of fuchsin and sodium, anomalous dispersions
27.1.52 Deviation with no dispersion
27.1.14 Liquid cell absorption
27.1.70 Scattering, Rayleigh scattering, Mie scattering, blue sky and red sun
4.114 Spectrum with a ray box, dispersion
25.00 Waves, wave motion
27.1.0 Electromagnetic radiation, spectrum
27.1.0 Electromagnetic radiation, visible spectrum
4.117 Absorption spectrum
27.1.16 Absorption spectrum of chlorophyll
27.1.15 Band absorption spectrum
4.132 Colours of sunlight, rainbow
3.9 Different colours (Primary)
27.5.2.1 Drink-can spectroscope
4.115 Emission spectrum
27.1.13 Filtered spectrum
4.118 Fluorescent lamp
4.116 Incandescent lamp
4.135 Infrared rays source
27.1.14 Liquid cell absorption
27.1.17 Metal films and dyes
27.4.3.5 Plotting the spectrum
27.1.80 Rainbows
27.1.11 Recombining the spectrum
4.114 Spectrum with a ray box, dispersion
3.11 Spin a colour disk (Primary)
4.136 Ultraviolet light source
27.2.0 Visible spectrum, rainbow
27.2.1 Infrared radiation (IR)
3.32.1 Composition of the atmosphere and greenhouse gases (See 2.)
37.43 Greenhouse effect in a model greenhouse, global warming
23.8.0 Heat transferred by radiation, black body radiation (See 2.)
23.8.12 Infrared radiation using iodine in alcohol
4.135 Infrared rays source
3.5.7 micron, µ, micrometre, µm, millimicron, nanometre, (See micrometre)
23.8.1 Thermoscope, simple thermoscope
27.5.0 Interference
27.5.0 Interference, Young's experiment
27.5.2.6 Babinet's principle
27.5.1.3 Cylindrical tube interference
27.5.2.1 Drink-can spectroscope
27.5.1.1 Double slits
27.5.1.2 Double slits and laser
27.5.1.4 Fresnel lens, lighthouse prism
27.5.2.0 Gratings, interference of polarized light, polarized sunglasses
27.5.2.5 Gratings in air and water
27.5.4.1 Michelson interferometer
27.5.2.4 Number of slits
27.5.2.7 Random multiple gratings
27.5.2.8 Speckle spots and random diffraction
27.5.2.9 Speckle patterns in arc light
27.5.2.3 Speckle patterns in unfiltered sun
27.5.2.2 Two dimensional grating
27.0.0 Light, wave or particle nature of light, speed of light
4.103 Light sources, producing light (experiments)
4.105 Light travels in straight lines, pinhole magnifier
4.103 Low voltage light source
4.104 Luminance and illuminance
27.4.0 Photometry, photometers, photoelectric cell, photoelectric effect
27.4.0 Photometry, photometers, photoelectric cell, photoelectric effect
27.4.1.5 Checker board
27.4.1.7 Foot-candle meter
27.4.1.4 Frosted globe surface brightness
27.4.1.2 Grease spot photometer, Bunsen grease spot photometer
27.4.1.6 Inverse square law model
2.2.4 Light bulb brightness, Joly photometer, wax block photometer
2.4.1 Make a photometer
27.4.1.1 Paraffin block photometer, Joly diffusion photometer
27.7.0 Photoelectric effect
27.4.1.9 Reflected surface brightness
27.4.1.3 Rumford shadow photometer
27.4.1.8 Surface brightness of a lens
27.6.0 Polarization
Order online: Polarized filters: 3 D movie glasses
27.6.4.0 Birefringence
27.6.3.0 Circular polarization
27.6.1 Dichroic polarization
27.6.1.3 Polaroid cut at 45 degrees
27.6.1.1 Polaroid on the overhead
27.6.2.0 Polarization by reflection
27.6.5.0 Polarization by scattering
27.6.1.2 Polarization mechanical model
27.5.3.0 Thin films
27.4.2 Radiation pressure, "light pressure"
27.4.2.1 Radiometer, quartz fibre radiometer
27.4.3.0 Black bodies, electric soldering iron, Stefan-Boltzman equation
27.4.3.1 Hole in a box, Bichsel boxes
27.4.3.2 Carbon block
27.4.3.3 Radiation from a black body
27.4.3.4 Good absorbers good radiators
27.4.3.5 Plotting the spectrum
27.4.3.6 Variac and light bulb
27.1.80 Rainbows
27.1.80 Rainbows
4.132 Colours of sunlight, rainbow
3.10 Rainbow colours (Primary)
27.2.0 Visible spectrum, rainbow
27.2.2 Ultraviolet radiation (UV)
Order online: UV Detection Beads, UV exposure, solar energy
Order online: Ultra Violet torch, ultraviolet light produces phosphorescence
7.2.2.1 Al, Aluminium properties, (See 3.)
18.7.13 Chlorine lost from swimming pools in sunlight
4.118 Fluorescent lamp
37.43 Greenhouse effect in a model greenhouse, global warming
37.39.1 Layers of the atmosphere, lapse rate
7.0.4 Necessary safety general knowledge
19.6.1 Paints, safety advice for paints and paint strippers, (See 3., 4.)
19.7.4 Sunscreens and sun-protective clothing
37.34 Solar ultraviolet radiation and skin cancer
4.136 Ultraviolet light source
See pdf: UV Sensitive Beads, UV detection beads
18.7.13 Chlorine lost from swimming pools in sunlight
18.7.13 Chlorine lost from swimming pools in sunlight
16.2.4.3.2 Chloramines in swimming pools
18.7.14 Cyanuric acid (CNOH)3, conditioner, stabilizer
18.7.18 Stabilized and unstabilized pools
18.7.23 Swimming pool terminology, (See: Trichlor)
27.6.4.0 Birefringence
27.6.4.0 Birefringence
27.6.4.9 Birefringent clear plastics
27.6.4.1 Calcite crystals
27.6.4.8 Colour with mica
27.6.4.7 Crystal structure of ice
27.6.4.4 Double refraction model
27.6.4.6 Nichol prism
27.6.4.3 Pendulum model
27.6.4.2 Plexiglass birefringence
27.6.4.5 Wavefront models
27.6.3.0 Circular polarization
27.6.3.5 Barber pole
27.6.3.6 Barbershop sugar tube
27.6.3.7 Faraday rotation
27.6.3.1 Laser and quinine sulfate, tonic water
27.6.3.4 Rotation by polarizing filter
27.6.3.2 Rotation by sugar solution
27.6.3.3 Three polaroids
27.6.2.0 Polarization by reflection
27.6.2.1 Black glass
27.6.2.2 Brewster's angle
27.6.2.3 Tilt the windowpane
27.6.2.4 Stack of plates
27.6.5.0 Polarization by scattering
27.6.5.4 Haidinger's brush
27.6.5.3 Polarization by scattering
27.6.5.1 Sunset with a polarizer
27.6.5.2 Tyndall experiment
27.5.3.0 Thin films
27.5.3.9 Absorption phase shift
27.5.3.6 Air wedge
27.5.3.4 Constant soap film
27.5.3.5 Cup with a soap film
27.5.3.7 Mica interference
27.5.3.2 Newton's rings
27.5.3.11 Oil film
27.5.3.1 Soap film interference
27.5.3.3 Stable black soap films
27.5.3.10 Tempering colours
27.5.3.8 Turpentine film
4.92 Diffraction in a ripple tank
Diffraction occurs when a straight wave passes through a narrow gap. The waves spread at the edge of obstacles, e.g. edges of a gap, and curve in behind an isolated obstacle.
1. Note diffraction when a wave hits two barriers separated by a gap of about 1 cm or less. Place the barriers 5 cm from the source of vibration, the vibrating beam. Block off the outer end of the barriers with side barriers. Increase the width of the gap to about 10 cm and note less diffraction. Put weights on the barriers if they start to vibrate.
2. Repeat the experiment with two equally separated gaps. Increase the width of the gap and note less diffraction.
4.119 Diffraction of light
Diffraction is the curving of light around the edges of objects and the consequent spreading of light when it passes through narrow gaps. Single slit diffraction pattern is different from double slit interference. Look at a vertical filament lamp through the slit formed by holding two fingers together.
4.134 Diffraction grating, spectroscope
See diagram 36.101: Spectroscope | See diagram 28.134.2: Diffraction grating
Order online: Spectator, spectrum kit, diffraction glasses
Order online: Diffraction Grating Film, single axis diffraction film, 500 lines / mm
Order online: Rainbow Glasses: Single Axis, 500 lines / mm diffraction grating glasses
Order online: Rainbow Peephole (diffraction gratings)
See pdf: Diffraction grating with Pringles tube spectroscope to produce spectrum
A diffraction grating is a piece of plastic or glass with many opaque parallel lines rules on it, e.g. 100 lines per mm, 300 lines per mm, 1000 per mm, 13 500 lines per inch. When light rays enter the spectroscope, they are separated, according to different wavelengths, into a spectrum or spectra and produce an interference pattern are sharpened to appear as bright lines of reinforcement (maxima). Each element has its own characteristic bright lines on its spectrum so the spectroscope is used for chemical analysis. Spectroscopes are also used in astronomy to determine the elements in the sun and stars because it can produce separated line images for light sources with similar wavelengths. The spectroscope invented by Joseph von Fraunhofer in 1820 used fine parallel wires.
1. Make a diffraction grating by drawing evenly-spaced clear black lines on a white card. Then take a high quality black and white photograph using a camera stand. Use the negative for a diffraction grating. However, you can also purchase cheap diffraction gratings as novelty spectacles, sometimes called "rainbow glasses".
2. Cut a 2 cm diameter round hole at one end of a cardboard shoe box. Attach a diffraction grating across the hole on the inside of the box. Note the direction of the slit on the grating. In the opposite side of the box, cut a 0.5 cm × 4.5 cm slit opposite the diffraction grating, with the longer side horizontal. Attach two razor blades to the outside of the slit, almost edge to edge, to form a very narrow vertical slit. Place a 12 V vertical filament lamp, e.g. a neon lamp or argon lamp, in front of the slit. Adjust the distance between the two razor blades so that you may see clear linear spectrums when you look through the round hole. Use the diffraction grating and a sharp source of light to see the order of colours in the spectrum. ROYGBIV, represents red, orange, yellow, green, blue, indigo and violet. Note the bright lines in spectra produced by fluorescent mercury lamps and neon signs.
3. Hold a feather near your eye and observe a burning candle far from you. Adjust the distance of feather from your eye until you see four X-shaped colour bands. You may also see two blue and two red bands in each of the four bands.
4. Stretch nylon gauze or a woman's fine scarf tightly and observe a burning candle through it. See colour stripes appearing in the direction of the fibres. Different weaving and different shapes of small holes will affect different shape of the stripes. You may see an X-shaped diffraction pattern through some types of nylon gauze.
5. Make a spectrum without a prism. Set a tray of water in bright sunlight. Lean a rectangular pocket mirror against an inside edge with the lower part immersed in the water. Adjust the mirror so that a spectrum appears on the wall.
6. Pass light through a spherical flask of water and view the rainbow on a screen placed between the light and the flask.
27.1.0 Electromagnetic radiation
See 19.3.5: Microwave cooking | See diagram 28.133: Electromagnetic spectrum
Type of
radiation		 γ-rays X-rays Ultraviolet rays Light rays
 Infrared rays Microwaves TV waves Radio waves
Approx. wavelength 0.01 × 10-9 m 10-9 m 0.1 × 10-6 m 0.4 to 0.7 × 10-6 m 0.01 mm 1 cm 1 m 1 km
Electromagnetic radiation can travel through a vacuum. Sunlight is electromagnetic radiation in all ranges. Light is electromagnetic radiation in all ranges, having a wavelength from 10-7 to 10-15 metres, including radio waves, infrared radiation, visible light, ultraviolet radiation, x-rays, and γ-radiation. The original "microwaves" were 1 mm to 15 cm wave length as required by developments in radar during the Second World War. In a narrow sense, light is only electromagnetic radiation in the visible range, having a wavelength from 400 manometers in the extreme violet to about 770 manometers in the extreme red. Light is considered to show both particle and wave properties. The fundamental particle or quantum of light is called the photon. The velocity of light in a vacuum is c = 2.997 924 58 × 108 m / s, or expressed in three significant figures, c = 3.00 × 108 m / sec. In transparent materials the speed of light is less than it is in a vacuum, e.g. 225 000 km / s in water, 200 000 km / s in glass, 29 9711 km / s in air. A medium in which the light velocity is low is called an optically dense medium. Light can be produced by a physical change, e.g. heating of an object or a chemical change, e.g. burning of magnesium.
Approximate wavelengths of radiation:
γ-rays < 1 × 10-11 m, have shortest wavelength and highest frequency, can kill cells and sterilize by killing pathogens
X-rays 1 × 10-11 to 1 × 10-8 m, can travel through soft tissue but not through bones or metal objects
Ultraviolet rays 1 × 10-8 to 4 × 10-7 m, can cause skin to form melanin pigment but excess can cause skin cancer
Visible light rays 4 × 10-7 to 7 × 10-7 m, can be analysed into a visible light spectrum
Infrared rays 7 × 10-7 to 1 × 10-3 m, given out by hot bodies
Microwaves 1 × 10-3 to 1 × 10-1m, generated by a magnetron in a microwave oven to cause vibration of molecules and speed cooking, a radio telescope can detect microwaves from distant stars not seen by visible light
TV and radio waves > 1 × 10-1m, have longest wavelength and lowest frequency, radar (radio detection and ranging) microwaves used for detection position of distant objects, e.g. aircraft
Visible spectrum with approximate ranges of wavelengths in nanometres, nm (1 nanometre = 10-9 m): violet 390 to 425 nm, indigo 425 to 445 nm, blue 445 to 500 nm, green 500 to 575 nm, yellow 575 to 590 nm, orange 590 to 620 nm, red 620 to 780 nm. The velocity of light in a vacuum, c = 3.00 × 108 m / second, but less in transparent materials, e.g. air 4.99 × 108 m / second, water 4.25 × 108 m / second, glass 4.00 × 108 m / second. The microwave region of the electromagnetic spectrum is from wavelengths 1 m to 1 mm. In Australia, microwave ovens operate at 12 cm wavelength.
27.1.0a Examine X-ray photographs
X-rays are a form of electromagnetic energy with high energy but very short wavelength which can pass through many substances normally opaque to visible light. The higher the frequency, i.e. the shorter the wavelength, the greater the penetrating power. The lower the atomic weight of a substance the more easily X-rays can pass through it. When X-rays pass through a human body, the fleshy parts produce a faint shadow on a photographic plate, the bones produce a darker shadow and any metal, e.g. a wedding ring, produces a very dark shadow. When discovered in Germany in 1895, their origin was unknown so were called X, but also called roentgen rays.
1. Examine animal and human X-ray photographs over a light table. If human X-ray photographs are cut into pieces, the students can practice reassembling them.
2. Compare inner and outer structures of animals, e.g. seashells, by comparing photographs with X-ray photographs.
1. Examine animal and human X-ray photographs over a light table. If human X-ray photographs are cut into pieces, the students can practice reassembling them.
2. Compare inner and outer structures of animals, e.g. seashells, by comparing photographs with X-ray photographs.
27.1.0b Radiation hazards
1. Radioactive substances usually emit α particles or β particles or γ-rays or combinations of these. X-ray units generate electromagnetic waves similar to γ-rays, but usually of lower frequency and longer wavelength. The amount and type of shielding needed depend on the penetrating power of the particular form of radiation. Sources of radiation are limited to sealed sources, radioactive chemical and mineral samples and high voltage electrical equipment.
The α- particles are charged and relatively heavy atomic particles so are easily stopped by a sheet of paper or the surface of the skin.
β-particles are stopped by a few millimetres thickness of aluminium or 2 cm of plastic material.
γ-rays have very short wavelength and are more penetrating and harder to stop. They are almost completely stopped by about 1 metre of concrete or about 5 cm of lead. Most will pass through the human body.
Medical X-rays are almost completely stopped by 3 millimetres of lead or 15 centimetres of concrete. X-rays pass through the body with some absorption depending on the density of organs, e.g. skin, bones.
2. Only teachers or laboratory staff are allowed to handle radioactive sources. Ionizing radiation in schools must only be used in simple experiments to demonstrate fundamental principles. The sources used and the methods of using them must be chosen to ensure that the degree of hazard is negligible. In school experiments involving X-rays or radioactive substances the radiation levels should be so low that no special shielding is required. However, it is important when using sources of radiation in schools to demonstrate the role of shielding as part of safe working practices. All radiation sources must be stored in separate lockable metal containers, e.g. a metal cash box, which are permanently labelled and kept in the school safe with access to authorized members of the school staff. Geological sample containing radioactive materials must be securely stored. Only minute amounts of radioactive materials are allowed to be kept in schools for demonstration purposes. The quantity of radiation absorbed by people during the short time they handle the equipment is negligible compared to the natural radiation. Never open a radioactive source or try to dissolve it in acid or other solvent. Radioactive sources at the end of their useful life must be disposed of according to government regulations.
3. Cold cathode discharge tubes may include a discharge tube with side tube for connection to a vacuum pump, Maltese cross discharge tube, discharge tube to illustrate the deflection of cathode rays by magnetic fields, windmill tube. These tubes are operated by high voltages produced by induction coils and may produce unwanted X-rays if operated at too high a voltage. Use the lowest possible voltage from the induction coil changing the distance of the make-and-break hammer from the iron core of the induction coil windings. Commence with the hammer well away from the core and use the adjusting screw to slowly decrease the distance between them until the discharge tube operates. Only teachers should operate a discharge tube, for a short a time as possible and with both teacher and student at a minimum distance of one metre from it.
27.1.01 Three conditions for colour
1.1 The colour must be in the source
1.2 The object must reflect or transmit the colour.
1.3 The detector must be sensitive to the colour
27.1.02 White light
It 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, due to 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.
27.1.03 Colours of object
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.
27.1.05 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.
27.1.06 True colour
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.
27.1.07 Primary colours
The colours red, yellow and blue (or violet) are called primary colours because they cannot be made by mixing other colours.
27.1.08 Secondary colours
These colours can be made by mixing two or more primary colours, e.g. orange, green, purple.
27.1.09 Fast colours
It refers to the colours of dyes which do not readily wash out in clothes washing water due to their chemistry.
27.1.1 Complementary colours
(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.
27.1.2 Subtractive colour effects
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.
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.
27.1.3 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 which 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.
27.1.4 See objects through coloured glass
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.
27.1.5 See flowers through monochromatic light
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.
27.1.6 White froth on a dark-coloured drink
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.
27.1.11 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.
27.1.12 Complementary shadow
Shadows of red and white lights illuminating the same object from different angles appear to produce green light.
27.1.13 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.
27.1.14 Liquid cell absorption
An absorbing solution is placed in a liquid cell placed in a beam of light before dispersion.
27.1.15 Band absorption spectrum
A flask of nitrous oxide is placed in the beam of white light before dispersion by a prism spectroscope. Didymium glass and dilute blood are also suggested.
27.1.16 Absorption spectrum of chlorophyll
Examine the absorption spectrum of chlorophyll obtained by macerating leaves in methyl alcohol.
27.1.17 Metal films and dyes
A thin film of gold transmits green but looks red yellow by reflection. Dyes also transmit and reflect different colours.
27.1.18 Dichromatism
Green cellophane transmits more red light than green. Stack lots of sheets and the colour of transmitted light changes from green to red.
27.1.19 Colour caused by absorption
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.
27.1.50. 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.
27.1.51 Dispersion curve of a prism
Light passes through a grating and then through a second slit at right angles and a prism generating a dispersion curve in colour on the screen.
27.1.52 Deviation with no dispersion
Light passed through oppositely pointed crown and flint glass prisms adjusted to give light deviated in two directions but with no dispersion. Light passes through prisms of crown and flint glass adjusted to give two beams of the same dispersion but different deviation.
27.1.53 Dispersion of fuchsin and sodium, anomalous dispersions
When salt is heated on a flame in the path of a narrow beam of light before dispersion the edges of the spectrum close to the dark band bend up or down.
27.1.70 Scattering, Rayleigh scattering, Mie scattering, blue sky and red sun
Rayleigh scattering is scattering of light and other electromagnetic radiation by polarized particles smaller than the wavelength of light, e.g. gas molecules, It is the main cause of the blue (blue-green hue) colour of the sky and the yellow colour of the sun where the shorter wavelengths of the spectrum, i.e. violet and blue, scatter more than the longer wavelengths. So the hue of the sun from within the atmosphere is a red-yellow, but redder near the horizon, caused by the remaining longer wavelengths while the shorter wavelengths are scattered away. Gas particles about the size of the wavelength of light cause Mie scattering to cause the white to grey colour of clouds. Rayleigh scattering allows a laser beam to be seen at night. Both Rayleigh scattering and Mie scattering are used to analyse solutions, gases and even biological tissue.
1. Red and blue beam
A red beam is passed through a solution of gum mastic but a blue beam is not.
2. Colour of smoke
Cigarette smoke is blue but after exhaling is white.
3. Multiple scattering in darkening of wet sand and whiteness of milk.
4. Dust halos
A glass plate covered with dust is held in a beam that converges into a hole in a screen. Circular halos appear on the screen around the hole.
27.1.80 Rainbows, spectrum
See diagram 28.220: Colour 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, fogbows, 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.
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 degrees. Use small glass spheres to generate bows and halos.
6. Arc lamp
An arc lamp directed at a sphere of water forms a rainbow on a screen rainbow
27.3.1 Single slit and laser
1. Shine a laser beam through single slits of various sizes.
2. Adjustable single slit
Look through a vernier calliper towards a monochromatic light 5 to 10 m distance. Look at a filament through a dark plate with a line scratched in it.
3. Single and double slits
Rule single and double lines on a photographic plate. Look at a line filament covered with half red and half blue filters.
4. Single and double slit projected
Focus a slit on the wall and place photographic plates with slits near the lens. For the single slit parallel lines are unevenly spaced. For the parallel slit pairs of lines of equal spacing are randomly spaced.
27.3.2 White light diffraction
1. A slit is projected on the wall and a second slit is placed at the focal point of the lens.
2. Electric razor detector sweep
A mirror mounted on an electric razor is used to sweep a diffraction pattern across a sensitive photodiode and the resulting pattern is displayed on an oscilloscope.
3. Diffraction about a circular object
A coin is placed between a pinhole and a screen. A small hole is punched in the screen in the shadow of the coin. While looking at the coin through the hole ring of light will be seen. Project the shadow from a point source onto a translucent screen.
4. Diffraction around knife edge
Slowly move a knife edge into a laser beam.
5. Diffraction pattern of a hair
Put a hair in a laser beam.
6. Shadow of a needle
A point source is placed behind a pair of needles.
7. Pass the razor blade
Hold a razor blade close to the eye so as to cut off part of an arc lamp.
8. Arago's (Poisson's) spot
Shine a laser beam at a small ball and look at the diffraction pattern. A laser beam is diffracted around balls.
27.4.0 Photometry, photometers, photoelectric cell
2.2.4 Light bulb brightness, Joly photometer, wax block photometer
Brightness and efficiency of light bulbs, photometer, luminance and illuminance, incandescent lamp, photoelectric cell, intensity of light, inverse square law, photoelectric exposure meter
27.4.1.1 Paraffin block photometer, Joly diffusion photometer
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.
27.4.1.2 Grease spot photometer, Bunsen grease spot photometer
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.
27.4.1.3 Rumford shadow photometer
Light sources are 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.
27.4.1.4 Frosted globe surface brightness
The surface brightness of a 40 W bulb is compared to a frosted globe placed over it.
27.4.1.5 Checker board
Use a point source to superimpose shadows of a rectangle and a 3h × 3w checkerboard rectangle.
27.4.1.6 Inverse square law model
A wire frame pyramid connects areas of 1, 4, and 16 units.
27.4.1.7 Foot-candle meter
Use a Weston type foot-candle meter to measure the inverse square law.
27.4.1.8 Surface brightness of a lens
Place the eye at the image point of a lens focussed on a dim lamp.
27.4.1.9 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.
27.4.2.1 Radiometer, quartz fibre radiometer
Focus a beam of light intermittently on a vane of the quartz fibre radiometer at the frequency of oscillation.
27.4.3.1 Hole in a box, Bichsel boxes
Holes in black boxes are blacker than the boxes. Two black boxes have blacker appearing holes in them. One box actually is painted white inside.
27.4.3.2 Carbon block
A carbon block with a hole bored in it is heated red-hot with a torch. The hole glows brighter. Bore a hole in an old carbon arc rod and heat electrically. The hole glows brighter.
27.4.3.3 Radiation from a black body
See 23.8.0: Heat transferred by radiation, black body radiation
Two holes are drilled in a carbon block. One is filled with a porcelain insulator and the block is heated red-hot with a torch. Graphite and porcelain heated red-hot look the same. A pattern on a porcelain dish shows brighter when heated.
27.4.3.4 Good absorbers good radiators
An electric element with chalk marks or china with a pattern are heated until they glow.
27.4.3.5 Plotting the spectrum
Measure the output of a thermopile as it is moved across a spectrum. Hold a thermopile connected to a galvanometer in different parts of a spectrum. Use a thermopile and galvanometer to show the infrared energy in the continuous spectrum.
27.4.3.6 Variac and light bulb
Vary the voltage to a 1 KW light bulb with a variac to show colour change with temperature. Vary the voltage across a clear glass lamp from zero to 50% over voltage Also measure the intensity and plot against power.
27.5.0 Interference, Young's Experiment
Two point sources of "in phase" light produce an interference pattern of nodes and antinodes just like two point dippers dipping in phase into water of uniform depth in a ripple tank. The shorter the wavelength, the more closely crammed is the interference pattern so the pattern for violet light is more crammed than the pattern for red light.
27.5.1.1 Double slits
Photograph two dark wires against a white background with high contrast film and use the negative for a double slit.
27.5.1.2 Double slits and laser
Shine a laser beam through double slits of different widths and spacing. Pass a laser beam through double slits of different widths and spacing. Direct a laser through a double slits of different dimensions. Pass a laser beam through double slits on the Cornell slide.
27.5.1.3 Cylindrical tube interference
The ring pattern from shining a point source down a reflecting cylindrical tube results from interference of two virtual sources.
27.5.1.4 Fresnel lens, lighthouse prism
See diagram 27.5.1.4: Lighthouse light, Freznel lens
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August-Jean Fresnel, 1788-1827, invented the Fresnel lens, lighthouse prism, to concentrate light from an oil lamp to form a beam. This prism is still used in lighthouses around the world and in some traffic lights. A conventional lens needed to focus the beam of a large lighthouse would weigh tonnes and be inefficient. However, the Fresnel "all surface" lenses do away with the useless middles of large lenses. Fresnel plano convex lenses may be formed in a thin sheet of acrylic plastic. However, they can start fires if left exposed to the sun. Fresnel also invented the Fresnel lantern containing prisms to produce a soft beam for for back light stage lighting or front light orchestra lighting.
27.5.2.0 Gratings, interference of polarized light, polarized sunglasses
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 to reduce glare caused by light reflected from polarizing surfaces, e.g. water, snow and sky radiation.
27.5.2.1 Drink-can spectroscope
Tape a replica grating over the hole cut a slit in the bottom. Make a slit in the cover of a film canister and place a grating over a hole in the bottom made with a No. 2 cork bore.
27.5.2.2 Two dimensional grating
View a motor car headlamp through a small square of silk.
27.5.2.3 Speckle patterns in unfiltered sun
Speckle patterns from sunlight scattered by a diffusing surface are common. Train yourself to see them.
27.5.2.4 Number of slits
Shine a laser beam through various numbers of slits with the same spacing.
27.5.2.5 Gratings in air and water
Measure the pattern of a laser beam incident on a diffraction grating placed inside an empty aquarium and with it full of water.
27.5.2.6 Babinet's principle
Carefully drawn black spots on white paper are photographically reduced and the positive and negative copies are used as complementary arrays.
27.5.2.7 Random multiple gratings
Exhale on clean glass to produce random multiple gratings of water droplets. Look through a drop of blood on a microscope slide at a point source or project onto a screen from a point source. Dust a bathroom mirror and hold a small light as close to the eye as possible. A collimated beam of white light is passed through a glass dusted with Lycopodium powder.
27.5.2.8 Speckle spots and random diffraction
The sparkling of a spot illuminated by a laser beam on the wall is caused by random interference patterns caused by scattered light speckle spots and random diffraction.
27.5.2.9 Speckle patterns in arc light
Speckle patterns can also be seen in arc lamp light The patterns disappear as the object is brought closer to the arc.
27.5.3.1 Soap film interference
Reflect white light off a soap film onto a screen. Project white light reflected off a soap film in a wire frame onto the wall. Illuminate a soap film with an extended source in a darkened room
27.5.3.2 Newton's rings
Reflect white light off Newton's rings onto the wall. Reflect light off a long focal length lens squeezed against a flat glass. Note change of ring size with different coloured light.
27.5.3.3 Stable black soap films
Vidal Sasson Extra Gentle Formula makes black films lasting five minutes or longer stable.
27.5.3.4 Constant soap film
Fit a large graduated cylinder with a rectangular frame with the handle protruding through the stopper. Fill half full with soap solution.
27.5.3.5 Cup with a soap film
Rotate a hemispherical shell with a soap film across the front so the black spot forms in the middle.
27.5.3.6 Air wedge
A sodium lamp illuminates an air wedge between two plates of glass. Diffuse sodium light with frosted glass before reflecting it off two plane glass plates. The diffused light from a high intensity sodium lamp is viewed by reflection off one and two pieces of plate glass glass plates in sodium light.
27.5.3.7 Mica interference
Examine interference by reflection of filtered mercury light from a mica sheet onto a screen. Reflect light from a mercury point source off a thin sheet of mica onto the opposite wall.
27.5.3.8 Turpentine film
White light incident of the surface of turpentine on water at an angle of 45-60 degrees is focussed on a screen.
27.5.3.9 Absorption phase shift
Cover the back of a microscope slide with streaks of an absorbing dye and observe under monochromatic.
27.5.3.10 Tempering colours
A thin film of oxide forms on a polished steel sheet when it is heated.
27.5.3.11 Oil film
The thickness of a film of oil on a pan of water that can be varied by sliding an iron bar across the surface for a variable interference filter.
27.5.4.1 Michelson interferometer
Use a Michelson interferometer with either laser or white light. Project coloured fringes from white light onto a screen insert a hot object in one path. Measure the power of solar cells in the two outputs of the Michelson interferometer.
27.6.1.1 Polaroids on the overhead
Examine polarization with two sheets of Polaroid and a pair of sunglasses on an overhead projector. Two Polaroid sheets are partially overlapped while aligned and at 90 degrees.
27.6.1.2 Polarization mechanical model
A pendulum is hung 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.
27.6.1.3 Polaroids cut at 45 degrees
Cut squares of Polaroid so the axes are at 45 degrees. Now turning one upside down causes cancellation.
27.6.2.1 Black glass
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.
27.6.2.2 Brewster's angle
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.
27.6.2.3 Tilt the windowpane
Reflect plane polarized light off a window pane and vary the angle of incidence through Brewster's angle.
27.6.2.4 Stack of plates
A stack of glass plated at 57 degrees will transmit and reflect light that is cross polarized.
27.6.3.1 Laser and quinine sulfate, tonic water
Pass a polarized laser beam through a cylinder filled with a quinine sulfate solution.
27.6.3.2 Rotation by sugar solution
Insert a tube of sugar solution between crossed Polaroids. Compare the rotation of plane polarized light in tanks containing sugar solution, turpentine and water.
27.6.3.3 Three Polaroids
Use 3 sheets of Polaroid on an overhead projector.
27.6.3.4 Rotation by polarizing filter
Stick a third sheet between crossed Polaroids.
27.6.3.5 Barber pole
A beam of polarized light is rotated 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.
27.6.3.6 Barbershop sugar tube
Illuminate a tube of corn syrup from the bottom. Insert and rotate a Polaroid filter between the light and tube.
27.6.3.7 Faraday rotation
Insert a partially filled glass container of wax into the core of a solenoid between crossed Polaroids.
27.6.4.0 Birefringence
Birefringence is having a different refractive index for light in different directions.
27.6.4.1 Calcite crystals
Use a second calcite crystal to show the polarization of the ordinary and extraordinary rays. Rotate a calcite crystal on an overhead projector covered except for a small hole Use a Polaroid sheet to check polarity. Rotate a calcite crystal with one beam entering and two will emerge one on axis and the other rotating around ordinary and extraordinary ray.
L-birefringent crystal
Project a hole in a strongly illuminated cardboard onto a screen through a calcite crystal. Interpose and rotate a polarizing plate to make the two images disappear alternately. Place a calcite crystal over printed material or a metal plate.
27.6.4.2 Plexiglass birefringence
Examine birefringence of a Plexiglas rod directly with a linearly polarized laser.
27.6.4.3 Pendulum model
Strike a pendulum with a blow then wait 1 / 4, ½ or 3 / 4 period and strike another equal blow at right angles to the first.
27.6.4.4 Double refraction model
A double pendulum displaced in an oblique direction will move in a curved orbit.
27.6.4.5 Wavefront models
See 2.0.5: Conic sections, ellipse
Wire models show spherical and elliptical wavefronts in crystals.
27.6.4.6 Nichol prism
One of a pair of Nichol prisms is rotated as a beam of light from an arc lamp is projected through Nichol prism.
27.6.4.7 Crystal structure of ice
A thin slab of ice is placed between crossed Polaroids crystal.
27.6.4.8 Colour with mica
Rotate a mica sheet between crossed Polaroids colour with mica quartz wedge.
27.6.4.9 Birefringent clear plastics
Doubly refraction material includes the following:
1. A crispy transparent cellophane wrapping for confectionery, potato chips and computer disk packets
2. Clear polystyrene rectangle from a window envelope
3. Polyethylene terephthalate (PET) soft drink bottle side.
Place a square cut from doubly refraction material between fixed and rotateable Polaroid sheets crossed so that transmitted light is extinguished. The 3 materials together let light pass through again!
PET bottles used to have a second black polyethylene as a cup on the bottom for a strong base but now have a bottom with five convolutions. PET is a birefringent 2-dimensional orientated plastic. A birefringent material has a different refractive index in the two dimensions, that is, a different speed for left-handed and right-handed helixes, slowing one down with respect to the other. When polarized light passes through, this differential results in a rotation of the plane of polarization. So crossed Polaroids are no longer crossed when material in between has caused a rotation. The amount of rotation depends on the degree of orientation of the polymer molecules and thickness of the film.
27.6.5.1 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.
27.6.5.2 Tyndall experiment
Shine light in one side of a box with a scattering solution and look at the scattered light out in a perpendicular direction.
27.6.5.3 Polarization by scattering
Add milk to water and show polarization of light scattered from a beam.
27.6.5.4 Haidinger's brush
Train yourself to detect polarized light with the naked eye.
27.2.0 Visible spectrum, rainbow
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.
27.7.0 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 which 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.