Light, Spectrum, Diffraction, Dispersion, Interfencece, Polarization.

School Science Lessons
(UNPh27) 2024-08-18

Spectrum, diffraction, dispersion, interference, polarization
Contents
27.1.0 CERN school experiments
27.2.0 Diffraction
27.3.0 Dispersion
27.4.0 Electromagnetic waves
27.5.0 Graphic file formats
27.6.0 Interference
27.7.0 Lasers
27.8.0 Magenta
27.9.0 Polarization
27.10.0 Radiation pressure
27.11.0 Spectroscope
27.18.0 Spectrum, Visible spectrum, Rainbows
27.13.0 Ultraviolet radiation

27.18.0 Spectrum, Visible spectrum, Rainbows
The visible spectrum or spectrum, a term invented by Isaac Newton, refers to the coloured band into which a beam of light is split when passing through a prism or diffraction grating.
The coloured band may contain bright or dark lines corresponding to frequencies emitted or absorbed.
The electromagnetic spectrum contains the entire range of frequencies of electromagnetic radiation from longest radio waves to shortest gamma rays, including the visible spectrum.
"Rainbow Glasses, Multi Axis", diffraction grating (toy product)
"Rainbow Glasses, Single Axis, 500 lines / mm diffraction grating (toy product)
"Rainbow Peepholes", diffraction gratings (toy product)
7.0 Colour, (Primary)
27.107 Primary and secondary colours
27.4.8 Visible spectrum, rainbow
37.2.20 Weather sayings (See: 2. Rainbow)
27.108 Coloured Pigments
27.7.4 Diffraction of light, diffraction grating, spectroscope (See: 2, 6)
12.1.11 Butyl chloride rainbow reactions
27.6.10 Oil on water, petrol on water, thin film interference
27.180 Rainbows
27.2.5 Spectroscope, diffraction grating, (See experiments: 1, 5)
Spectrum, Rainbows, visible spectrum

27.13.0 Ultraviolet radiation
18.7.13 Chlorine lost by ultraviolet radiation, (swimming pools)
35.5 Colour (Geology)
23.8.21 Colour temperature
2.3 Blueprints and diazo prints, (Experiment)
27.4.1 Electromagnetic radiation
16.3.5.8 Fluorescence spectroscopy of quinine
37.1.5 Solar ultraviolet radiation and skin cancer
27.4.7 Ultraviolet rays
38.8.1 Ultraviolet radiation, Gas discharge tubes

27.2.0 Diffraction
27.2.1 Random multiple gratings diffraction
27.2.2 Resolution, resolvance of diffraction grating, spectrometer, microscope, telescope
27.2.3 Single slits and double slits diffraction
27.2.4 Speckle spots and random diffraction
27.2.5 Spectroscope, diffraction grating
27.2.6 White light diffraction

27.3.0 Dispersion
Experiments
27.3.1 Band absorption spectrum, nitrous oxide, dispersion
27.3.2 Deviation with no dispersion
27.3.3 Dispersion curve of a prism
27.3.4 Dispersion of fuchsin and sodium, anomalous dispersions
27.3.5 Liquid cell absorption dispersion
27.3.6 Scattering, Rayleigh scattering, Mie scattering

27.4.0 Electromagnetic waves, electromagnetic spectrum
28.133: Electromagnetic spectrum (gif)
27.4.1 Electromagnetic radiation
27.4.2 Gamma rays, γ-rays
27.4.3 Microwaves
27.4.4 Plotting the spectrum
27.4.6 Coherer effect
27.4.7 Ultraviolet rays
27.4.8 Visible spectrum, rainbow
Experiments
27.3.1 Band absorption spectrum, nitrous oxide, dispersion
27.3.5 Liquid cell absorption dispersion
27.4.11 Visible light rays
27.4.12 X-rays

27.6.0 Interference, Young's experiment
27.6.1 Interference, Young's two-slit experiment, (double-slit experiment)
27.6.2 Absorption phase shift, monochromatic light
27.6.3 Air wedge interference
27.6.4 Babinet's principle of diffraction
27.6.5 Colours of soap films
27.6.6 Cylindrical tube interference patterns, coherent light / incoherent light
27.6.7 Fresnel lens, lighthouse prism
27.6.8 Interference of polarized light, polarized sunglasses
27.6.9 Newton's rings with float glass
27.6.10 Oil on water, petrol on water, thin film interference
27.2.1 Random multiple gratings interference
27.7.8 Soap film interference
27.7.9 Speckle patterns in arc light interference
27.6.14 Tempering colours
27.6.15 Turpentine film
27.6.16 Wave front models
27.4.3 Microwaves
27.4.3.1 Microwaves
27.4.3.2 Microwave containers
27.4.3.3 Microwave cooking
27.4.3.4 Microwave radiation
27.4.3.5 Superheating in a microwave oven

27.9.0 Polarization, Polaroid film
27.9.1 Dichroic polarization colours
27.9.2 Haidinger's brush
27.9.3 Polarization by scattering

27.10.0 Radiation pressure, "light pressure"
27.52 Holes in carbon blocks
27.4.4 Plotting the spectrum
27.56 Variable autotransformer (Variac) and light bulb

27.11.0 Spectroscope
27.8.0 Magenta
27.2.5 Spectroscope, diffraction grating (Experiments)
27.194 Spectroscope for materials analysis, shoe box spectroscope
27.195 Atomic Absorption Spectroscopy (AAA)
Deuterium

27.7.0 Lasers
27.7.1 Lasers, coherent light, Young's two-slit experiment
27.6.4 Babinet's principle of diffraction
27.9.1 Dichroic polarization colours
27.7.4 Diffraction of light (See: 11.)
27.6.1 Interference, Young's two-slit experiment, (See: 1. - 3.)
27.7.6 Michelson interferometer
27.3.6 Scattering, Rayleigh scattering, Mie scattering
27.2.3 Single slits and double slits diffraction
27.7.8 Thin film interference, soap film interference, (CDs)
27.7.9 Speckle spots and random diffraction
27.2.6 White light diffraction
26.4.3 Compact disc, CD
28.2.19 Corner cube, corner reflector, retrodirective mirrors
38.6.4 Diodes, laser diodes
7.2.0 Laser safety
28.1.1 Speed of light

27.2.5 Spectroscope, diffraction grating
See diagram 27.2.5.1: Spectroscope
See diagram 27.2.5.2: Diffraction grating
See diagram 27.101: Colour wheel
A diffraction grating is a piece of plastic or glass with many opaque parallel lines rules on it.
For example: 100 lines per mm, 300 lines per mm, 1000 per mm, 13, 500 lines per inch.
Light rays entering a spectroscope are separated to wavelengths, in a spectrum for an interference pattern, 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 used in astronomy to determine the elements in the sun, 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.

27.6.5 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.

27.4.1 Electromagnetic radiation
27.4.3.3 Microwave cooking
See diagram 28.133: Electromagnetic spectrum
See Table 7.28: Types of radiation in the electromagnetic spectrum
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.
They include 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.
Light is electromagnetic radiation in the visible range, with a wavelength from 700 nanometres for red light to 400 nanometres for violet light.
(1 nanometre, nm = 0.000000001 m)
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.

See diagram 4.133: Electromagnetic radiation
All electromagnetic radiation travels at the speed of light, c = 3 X 108 m s-1.
The visible light spectrum has wavelengths 0.7 X 10-6 metres, red, to 0.4 X 10-6 metres, violet.
Approx. wavelengths of radiation:
Gamma rays < 1 x 10-11 m
0.01×10⁻¹² meters 10×10⁻¹² meters X-rays >1 x 10-11 to 1 x 10-8 m
Ultraviolet rays 1 x 10-8 to >4 x 10-7 m
Visible light rays >4 x 10-7 to >7 x 10-7 m
Infrared rays >7 x 10-7 to 1 x 10-3 m
Microwaves 1 x 10-3 to >1 x 10-1 m
TV and radio waves >> 1 x 10-1m.
Visible spectrum with approximate ranges of wavelengths in nanometres, nm (1 nanometre = 10-9 m):.
violet 390 to 425 nm, indigo 425 to 445 nm, blue 445 to 500 nm, green 500 to 575 nm, yellow 575 to 590 nm, orange 590 to 620 nm, red 620 to 780 nm.
The velocity of light in a vacuum, c = 3.00 X 108 m / second, but less in transparent materials: air 2.99 X 108 m / second, water 2.25 X 108 m / second, glass 2.00 X 108 m / second.
The microwave region of the electromagnetic spectrum is from wavelengths 1 m to 1 mm.
In Australia, microwave ovens operate at 12 cm wavelength, 2.45 gigahertz, (GHz).

27.4.2 Gamma rays, γ-rays
Gamma rays, γ-rays, have shortest wavelength, highest frequency, most energy of any part of the electromagnetic spectrum.
They are ionizing radiation and so are hazardous, because they can kill cells and sterilize by killing pathogens.
They are produced by neutron stars, pulsars, supernova explosions, nuclear explosions on Earth, lightning, and radioactive decay of natural radioisotopes.
They are not reflected by mirrors, because their wavelengths are so short they pass through the space between most atoms.
However, when they collide with electrons in certain crystals, they scatter, Compton scattering, and the slowed particles can be detected.

27.4.12 X-rays
Be careful! X-rays should not be used in schools!
X-rays, Roentgen rays (discovered by Wilhelm Röntgen, Germany, 1845 -1923), have very small wavelengths and act more like particles than waves.
X-rays are able to penetrate soft tissue. but not bone.
An X-ray image shows the bones as a shadow.
Bones and teeth, being more dense, absorb more X-rays than skin, so shadows of bones can be projected onto film.
They are used for customs and security detection, and testing pearls, gemstones, old paintings, welding joints.
They kill cancer cells, but healthy tissue must be shielded from them.
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.
Experiments
Examine 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.
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.
3. Compare inner and outer structures of animals, e.g. seashells, by comparing photographs with X-ray photographs.

27.4.7 Ultraviolet rays
Ultraviolet rays from the Sun are subdivided into UV-A, UV-B, and UV-C.
UV-C rays are the most harmful. but are almost completely absorbed by the atmosphere.
UV-B rays cause sunburn and increase the risk of damage to DNA.
About 95 % of UV-B rays are absorbed by ozone in the Earth's atmosphere. but each year, a "hole" of thinning atmospheric ozone expands over Antarctica to exposing people increased levels of harmful UV rays.
Vitamin D is synthesized by people exposed to adequate sunlight.

27.4.11 Visible light rays
Visible light rays, 4 × 10-7 to 7 × 10-7 m, can be analysed into a visible light spectrum.
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.

27.26 Infrared rays
Infrared rays are emitted by hot bodies.
At about 500oC hot body becomes "red hot" and emits infrared rays and red light.
At about 1000oC electric lamp filaments become "white hot" and emit infrared rays an all colours of the spectrum.
Infrared lamps are used to treat sore muscles and dry paint.
Television remote control used infrared radiation.
Infrared photography allow detection of hot bodies in the dark.

27.4.3.1 Microwaves
Microwaves are a form of high frequency radio waves as in AM and FM radio, but are much shorter.
A magnetron tube, the heart of the microwave oven, converts electricity into microwave energy.
From the magnetron tube, microwave energy is transmitted to the oven cavity through a small plastic-covered piece.
In the food, the microwaves are converted to heat, losing half their power every 2-3 cm.
Metals reflect microwave energy, but do not become hot.
Plastic, paper and glass transmit microwave energy, but do not become hot.
Food absorbs microwave energy, causing the water molecules in the food to vibrate.
Friction from the vibration produces heat energy and so causes the food to become hot.
Microwaves 1 × 10-3 to 1 × 10-1m, generated by a magnetron in a microwave oven.
It causes vibration of molecules and speed cooking.
A radio telescope can detect microwaves from distant stars seen by visible light.
The microwave region of the electromagnetic spectrum is from wavelengths 1 m to 1 mm.
In Australia, microwave ovens operate at 12 cm wavelength.
Besides cooking, a microwave oven can be used to sterilize soil in potting mix, extract more juice from squeezed citrus fruit, liquidized. hard honey, warm damp plates and dry herbs and flowers.
Small insects can survive in a microwave oven, but larger creatures, e.g. mice, cats, humans, can be killed by microwave oven radiation.
The cause of death is the heating of the brainstem, which controls breathing, swallowing, and other autonomic physical reactions.
1. The four basic methods of cooking are as follows:
* Wet heating: boiling, steaming, pasteurization 100oC to 120oC,
* Dry heating: baking, roasting, up to 250oC,
* Hot oil frying, up to 300oC,
* Microwave cooking, up to 120oC.
The temperature of meat in a microwave oven is never above 100oC, so no maillard reaction can occur.

27.4.3.4 Microwave radiation
1. Microwaves are a form of high frequency radio waves similar to the waves used by AM and FM radio.
Microwaves enter from the outside of food losing about half their energy every 3 cm.
Microwaves are reflected by metals so the walls of the microwave oven are made of stainless steel or epoxy coated stainless steel.
The microwaves can bounce around the inside and not escape into the kitchen.
Food is usually heated on a revolving turntable to produce an even heating and avoid hot spots and cold spots.
The holes in the steel door are too small to let the microwaves pass through.
Microwaves are transmitted by paper, glass and some plastics that do not absorb microwave energy and do not become hot.
Microwaves are absorbed by food containing moisture.
2. Microwave region of the electromagnetic spectrum is from frequency 300 MHz, wavelength 1 m, to 300 000 MHz, wavelength 1 mm.
Australia and UK, microwave ovens operate at 2450 MHz (Class B / Group 2, ISM equipment Standard CISPR11), wavelength 12 cm.
In other places, the microwave oven wavelength may be 12.24 cm.
This is a lower frequency and lower energy than ionizing radiation.
For safety reasons, when the microwave door is opened, switches must cut the power.
The power flux density of microwave radiation should not exceed 50 W / M2 at any point 5 cm or more from the oven external surface
The inside of a microwave oven is coated with metal that acts like a Faraday cage and reflects the microwaves (radio waves).
3. Microwave radiation is generated in an electronic tube called a magnetron.
High voltage emits electrons from a heated cathode that are diverted by magnets to spin in a circular orbit.
This causes metal plates to resonate and emit microwaves to be conducted into the interior space of a microwave oven.
The patterns of more intense radiation can be seen by placing thermal paper used for thermographic imaging at different heights in the oven.
However, the same thermal paper placed on the turning turntable, (carousel), shows a more equal distribution of the radiation.
4. Water molecules are electric dipoles with a positive charge at one end and a negative charge at the other end.
They spin when they align with the alternating electric fields in the microwave oven.
However, a block of dry ice is not melted in a microwave oven, because the carbon dioxide molecule is not a dipole.

27.4.3.3 Microwave cooking
Microwave cooking occurs, because water is a strong absorber of microwaves, so the water in food boils and turns to steam.
In a normal oven, a range of black body radiation is generated, but only some longer sections of this radiation are absorbed by food.
The radiation penetrates the surface of the food for about one wavelength for infrared heat radiation < 1 / 10 mm.
Only the outside of the food is directly affected and the inside of the food is heated by conduction.
The 12 cm wavelength microwaves penetrate deeper into the food.
Avoid exploding food, e.g. tomatoes, apples, by cutting into them before microwave cooking.
Avoid dry food with spikes or sharp corners, e.g. pieces of sweet potato or broccoli, because electrical arcing may occur between the spikes
This may cause charring of the food.
Elevate the cooking pan in the microwave oven to allow more bounce of microwaves up from the floor of the microwave oven to get even cooking.
Microwaves are not used for defrosting, because ice has few free molecules of water to be excited by the microwaves.
Microwave ovens do not heat food by operating at a special resonance of water molecules in the food.
A microwave oven could operate at many frequencies.
The 22 GHz resonant frequency of isolated water molecules has a wavelength too short to penetrate common foodstuffs to useful depths.
The typical oven frequency of 2.45 GHz was chosen partly due to its ability to penetrate a food object of reasonable size.
, Microwave ovens do not cook food from the inside out, because 2.45 GHz microwaves can only penetrate approximately 1 cm into most foods.
The inside portions of thicker foods are mainly heated by heat conducted from the outer portions.
Microwave ovens cannot cause cancer, because as microwave radiation is non-ionizing radiation.
Do microwaves make food radioactive?
The energy of a microwave photon in a microwave oven is 0.00001 eV.
The but the minimum ionization energy of common chemical substances is about 0.01 eV.
So microwave ovens do not cause chemical changes from radiating food.
Photons in an infra-red oven are much higher energy, but energy required to change a nucleus to make food "radioactive" is many powers of ten higher.

27.4.3.2 Microwave containers
Microwave cooking containers should not absorb the radiation.
They should be made from material with low polarity, i.e. low dielectric constant and low dissipation factor at 2450 MHz.
The material should be stable at high temperature and have good resistance to oil vapour, e.g. polystyrene and certain glass.
Never use metal containers or plates decorated with metal leaf in microwave ovens.
Microwaves cannot pass through metal and may be reflected to damage the magnetron.
Remove meal foil packaging and twisted ties or tags.
Do not use dishes with metallic rims, cups with glued-on handles, delicate glassware, cut glass vases, jars with metal lids.
However, you can use metal foil to protect food that might overcook in some areas.
Do not cook in plastic food storage bags, because they may melt.
However, "GLAD WRAP" may be used to cover dishes, but do not remove it immediately after cooking.
Do not use brown paper bags or newspaper, because they contain metallic impurities that may cause blue spark arcing and damage the oven.
Do not put mercury thermometers in the microwave oven.
Do not put a wet cat in the microwave oven.
Use containers labelled "microwave oven safe".
Use shallow round dishes, because microwaves penetrate only 2 to 3 cm into most food.
Food in the corners or dishes with corners will receive more energy and probably overcook.
Arrange food containers in a ring at outside of the rotating turntable.
Put the container containing thin food on top of another container on the turntable to get more bounce of microwaves through the food.
Remember that if food is not cooked through, e.g. poultry, then bacteria may remain unharmed inside.
Examples of microwave cooking containers:
* "Starmaid" baking dish, plastic, 1 litre with lid, 190 mm × 170 mm × 68 mm.
* "Décor Thermoglass", for conventional ovens and microwaves, with microwave safe lid with a steam release vent, 1 litre.
* Microsafe milk heating jug, "Microsafe" jug has cone-shaped lid if it boils, the milk flows up though the cone, safely back into the jug.
* Melamine crockery, green, virtually unbreakable, dishwasher safe.
Not suitable for microwave oven, plate, 18 mm diameter.

27.4.3.5 Superheating in a microwave oven
When tap water is heated on a stove top, the maximum superheating is 100.75oC.
However, in a microwave oven, superheating temperatures of 105oC to 110oC can occur.
The microwave oven heats the water directly and the container indirectly.
This is the opposite of what happens when heating a container of water on a stove.
In the microwave oven the water is not heated near the beaker surface where nucleation is most likely to occur.
Evaporation occurs only at the water surface and is not fast enough to cool the bulk water.
There is much less superheating in plastic cups, because water does not wet plastic and many more nucleation sites are available for boiling.
It is dangerous to put a cup of still water in the microwave oven
When you try to remove the cup nucleation may occur and the water boils, forms bubbles and steam, and may scold your hand.
To avoid the delayed eruptive boiling of liquids after cooking, leave to stand for at least 20 seconds before removing food or liquids from the oven.
This delay allows for some cooling and more even distribution of heat.
If a plastic fork or tea bag is placed in the liquid before the oven is turned on, they provide nucleation sites for boiling to occur, so no superheating.
Experiments
1. Test a container for potential microwave oven use.
Fill a one cup glass measure with water and put it in the microwave oven alongside the container to be tested, e.g. a ceramic dish.
Heat for on high for 60 seconds.
If the container is microwave oven safe, it will remain cool, but the water should be hot.
However, you cannot use this test on plastic containers that must be labelled microwave "oven safe".
2. Cook a potato for 2 minutes then cut it open to see the translucent areas finger-like regions extending from the outside.
This is where the potato has been heated above 60oC.
This show that microwave ovens do not provide a uniform density of microwave energy.
So "hot spots" and "cold" spots may exist in microwave ovens, unlike the conventional oven.
This phenomenon is the reason cooks suggest leaving food cooked by microwaves to "stand" for some time after cooking.
This delay allows the temperature to equalize in the food.
3. Compare popcorn cooked in the microwave with popcorn cooked on the conventional stove.
Use the same number of corn (maize) grains and later count the number of unpopped grains.
Carefully open popcorn bags away from your face!.
4. Do not put sealed tins or glass jars in a microwave oven, e.g. babies bottles fitted with a screw cap.
Remove food from tin cans.
If boiling liquids in a microwave oven use a wide moth container.
Also, puncture skins of potatoes, apples, sausages and oysters to allow steam to escape.
Food with a high fat content, e.g. sausage roils, may catch fire if overcooked.
5. Eggs cooked in the microwave oven.
Eggs may explode, so avoid exploding food by cutting into eggs before microwave cooking.
Egg yolk contains fat so tends to cook more quickly than egg white.
Even out of the shell, eggs may explode in the microwave, because rapid heating causes a build-up of steam.
Before cooking puncture egg yolks and whites.
6. Experiment
Observe the effect of the wavelength of microwaves.
Take out the turntable and put on it a one cup glass measure with water and a long stick of uncooked spaghetti.
Heat at high for one minute.
The spaghetti will have an uncooked area every 12 cm.

27.4.6 Coherer effect
A coherer, Branly coherer (invented by Eduard Branly, 1844-1940, France), is a glass tube containing metal filings.
It decreases in resistance to an electric current and so was used to detect radio waves.
The metal filings cohere to reduce resistance of the bulk filings.
By tapping the glass tube, the metal filings can be made to fall apart or "decohere".
The on-off activity was used to transmit the first morse code signals.
Experiment.
Make a simple coherer by putting some iron filings into a cleaned-out narrow plastic tube reservoir from a ballpoint pen.
Inserting screws in each end to act as electrodes.
Complete the circuit with a battery and an LED.
A signal from a transmitter, e.g. an auto ignition coil or a barbecue piezo ignitor, turns on the LED, and tapping the coherer turns it off.

27.7.1 Lasers, coherent light
Coherent light is light that is "in phase".
A laser (Light Amplification by Stimulated Emission of Radiation) is a powerful source of coherent light.
See diagram 27.6.1: Young's two-slit experiment.
Two point sources of coherent light can be produced by directing any light through a single narrow slit then through a double slit arrangement.

27.2.3 Single slit diffraction and double slit diffraction
Diffraction (Commercial).
See diagram 27.2.3: Single slit diffraction.
See diagram 27.6.1: Coherent light, Double slit diffraction.
Diffraction is both the curving of light around the edges of objects and the spreading of light when it passed through a narrow slit.
The pattern of single slit diffraction is different from double slit diffraction in that the central reinforcement is twice as wide as all the other reinforcements.
Also, the intensity of light regions diminishes at places further away from the central reinforcement.
For any nodal point in the single slit diffraction pattern, sin θ= n λ / ω = x / L, where n = the number of the nodal lines.
Experiments
1. Adjustable single slit.
Look through a vernier caliper towards a monochromatic light 5 to 10 m distance.
Look at a filament through a dark plate with a line scratched in it.
2. 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.
3. 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.
4. Turn on the laser.
The pattern appears on the wall.
The slit widths and spacing are printed on the slide.
To select different slits, slide the holder sideways.
A Cornell slit film card includes double slits of increasing width and spacing.
5. Turn on the laser and place the slit slide in front of the beam so the beam hits one of the many slits on the slide.
View the results of the interference pattern on the wall.
The lights in the room may need to be darkened.

27.2.6 White light diffraction
Diffraction (Commercial).
Experiments
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 photo diode.
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. Pass the razor blade.
Hold a razor blade close to the eye so as to cut off part of an arc lamp.
5. Shadow of a needle.
A point source is placed behind a pair of needles.
6. Diffraction around knife edge.
Slowly move a knife edge into a laser beam.
7. Diffraction pattern of a hair.
Put a hair in a laser beam.
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.
9. Turn on the laser.
The pattern appears on the wall.
The slit widths are printed on the slide.
To select a different slit, slide the holder sideways.
A Cornell slit film card includes a slit of gradually increasing width.

27.33 Diffraction in a ripple tank
Diffraction (Commercial).
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.
Experiments
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.

27.7.4 Diffraction of light, diffraction grating, spectroscope
Diffraction (Commercial).
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.
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.
A diffraction grating with 15000 lines to the inch, mounted vertically in the beam produces a brilliant spectrum.
When light rays enter the spectroscope, they are separated, according to different wavelengths, into a spectrum or spectra.
They produce an interference pattern 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.
They can be used to produce separated line images for light sources with similar wavelengths.
The spectroscope, invented by Joseph von Fraunhofer in 1820, used fine parallel wires.

"Diffraction Grating Films", single axis diffraction film, 500 lines / mm (toy products)
"Rainbow Glasses, Multi Axis", diffraction grating
"Rainbow Glasses, Single Axis, 500 lines / mm diffraction grating
"Rainbow Peepholes", diffraction gratings
"Spectator glasses, diffraction glasses with blue and yellow filters
See diagram 36.101: Shoe box spectroscope
See diagram 28.134.2: Diffraction grating
Experiments
1. A slide projector is directed towards the front of the lecture theatre.
A diffraction grating with 15000 lines to the inch is mounted vertically in the beam producing a brilliant spectrum in the region shown.
2. 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".
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.
7. 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.
8. Make a 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.
9. View a motor car headlamp through a small square of silk to observe two-dimensional grating diffraction interference.
10. Train yourself to see speckle patterns in unfiltered sun diffraction interference scattered by a diffusing surface.
11. Use a diffraction grating 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.

27.52 Holes in carbon blocks
1. 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.
2. 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.

27.54 Good absorbers good radiators
An electric element with chalk marks or china with a pattern are heated until they glow.

27.4.4 Plotting the spectrum
Experiments
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.

27.56 Variable autotransformer, (Variac) and light bulb
See diagram 27.56: Variac circuit.
A variable autotransformer (autostep down transformer), e.g. Variac, is used to vary the output voltage for a steady AC input voltage.
Variable autotransformers, often called "Variacs" provide a voltage-adjustable source of alternating current (AC) electricity.
Unlike a transformer with isolated input and output windings, variacs have a single winding that serves as both the input and output source.
Variacs do not provide isolation from the line voltage, unlike a true transformer, so they are not suitable for use with low voltage devices.
They are used for equipment testing and repair, not as a permanent source of power.
Experiment.
Vary the voltage to a 1 KW light bulb with a variable autotransformer 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.

27.6.1 Interference, Young's two-slit experiment (double-slit experiment)
Young's Slits (Commercial).
See diagram 27.6.1: Young's two-slit experiment.
Young's two-slit 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.
1. For any point in the pattern observed, sin θ = (path difference / d) = (x / L).
2. For any nodal point in the pattern observed, sin θ = (n - 1/2) λ / d = (x / L).
3. The bright bars in the interference pattern observed, and the nodes, are uniformly spaced, x.
x = Lλ / d.
4. The shorter the wavelength, the more closely crammed is the interference pattern.
So the pattern for violet light, λ = 4.5 X 10-5 m is more crammed than the pattern for red light, λ = 6 X 10-7 m.
Experiments
1. Shine a laser beam through single slits of various sizes.
2. 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.
3. Shine a laser beam through various numbers of slits with the same spacing.
4. Photograph two dark wires against a white background with high contrast film and use the negative for a double slit.

27.6.3 Air wedge interference
See diagram 27.6.3: Air wedge.
An air wedge is formed by placing a very thin object between two flat glass plates so that the two plates of glass touch at one end and are separated by a small distance at the other end.
Directing parallel, monochromatic light at right angles to the top flat plate produces a series of parallel bands of maxima and minima when viewed from above caused by the thin film interference of reflected light from both the upper and lower surfaces of the air wedge.
Interference occurs between light reflected from the bottom surface of the top glass plate and light reflected from the top surface of the bottom glass plate to form bright and dark interference fringes by constructive and destructive interference.
Experiment.
1. Place a thin object between one edge of two perfectly flat glass plates.
Direct parallel monochromatic light normally onto the upper glass plate to observe from above a series of parallel bands of maxima and minima, caused by thin film interference of reflected light from the upper and lower surfaces of the air wedge.
2. 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.

27.6.6 Cylindrical tube interference patterns, coherent light / incoherenrt light
Coherent light has light waves with a constant phase relationship to each other.
Incoherent light have phase relationships that vary randomly.
A point source of coherent light illuminating the inner walls of a cylindrical tube causes interference fringes on a screen placed at the end of the tube.
The ring pattern from shining a point source down a reflecting cylindrical tube results from interference of two virtual sources.
Reflection of coherent light from a cylindrical glass tubing gives many interference fringes which extend around the tube for more than 180o from the incident beam direction.
These fringes have a spacing which varies with angle.
The cause of this interference pattern is the superposition of the reflections from the outer and inner surfaces of the cylinder.
Cylindrical tube interference patterns are being used for research in the non-invasive imaging of blood and lymphatic vessels of living organisms.

27.6.7 Fresnel lens, lighthouse prism
"Fresnel Lens", versatile lens for classroom, unbreakable (industrial product).
See diagram 27.5.1.4: Lighthouse light, Fresnel lens
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 back light stage lighting or front light orchestra lighting.

27.6.8 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.

27.2.2 Resolution, resolvance of diffraction grating, spectrometer, microscope, telescope
Diffraction (Commercial).
Diffraction causes images of closely-spaced objects to merge with the loss of individual detail, called poor resolution.
Increased magnification only produces a large image lacking in detail.
A diffraction grating has a high resolving power, so is used in analysis of a spectrum, because it can produce separated line images for light sources with similar wavelengths.
The "resolving power", resolvance, of a device used to separate the wavelengths of light is as follows:.
R = λ /delta λ, where delta λ = smallest resolvable wavelength difference.
The two sodium "D-lines" are at 589.00 nm and 589.59 nm, so R = 589 / 0.59 = 1000, the standard benchmark for the resolvance of a diffraction grating or spectrometer.
The resolving power of a microscope or telescope is a measure of its ability to produce separated images.

27.5.0 Graphic file formats
1. JPEG (Joint Photographic Experts Group) is the most common format for storing and transmitting photographic images on the World Wide Web.
The degree of compression can be adjusted, allowing a selectable trade-off between storage size and image quality, typically 10: 1 compression with little perceptible loss in image quality.
JPEG compression is used in a number of image file formats.
2. TIFF (Tagged Image File Format) format is a flexible format that normally saves 8 bits or 16 bits per colour (red, green, blue) for 24-bit and 48-bit totals, respectively.
TIFF image format is not widely supported by web browser.
The PNG (Portable Network Graphics) file format was created as the free, open-source successor to the GIF.
The PNG file format supports true colour (16 million colours) while the GIF supports only 256 colours.
The PNG file excels when the image has large, uniformly coloured areas.
The loss less PNG format is best suited for editing pictures, and the glossy formats, like JPG, are best for the final distribution of photographic images, because JPG files are smaller than PNG files.
3. GIF (Graphics Interchange Format) is limited to an 8-bit palette, or 256 colours so is suitable for storing graphics with relatively few colours such as simple diagrams, shapes, logos and cartoon style images.
4. BMP file format (Windows bitmap) handles graphics files within the Microsoft Windows OS.
Typically, BMP files are uncompressed, hence they are large; the advantage is their simplicity and wide acceptance in Windows programs.

27.6.4 Babinet's principle of diffraction
Diffraction (Commercial).
See diagram 27.6.4: Light diffracting around a hair.
Babinet's Principle states that the diffraction pattern for an aperture is the same as the diffraction pattern for an opaque object of the same shape if illuminated in the same manner.
So the diffraction pattern from an opaque body is identical to diffraction pattern from a hole of the same size and shape except for forward beam intensity.
Except for the intensity of the central spot, the pattern produced by a diffracting opening of any shape is the same as the pattern produced by its conjugate.
A circular hole and a same-sided droplet will produce the same diffraction pattern.
Observing the diffraction from shining a laser through a smear of blood cells can be used to determine the size of the cells.
Babinet's Principle can be used to measure particles in the corona around the sun or moon and smog particles, and diffraction from a hair in a light beam.
Babinet's principle is used to find equivalence in size and shape, e.g. size of red blood cells by comparing diffraction pattern with an array of small holes.
Babinet's principle is used in slot antennas at frequencies between 300 MHz and 24 GHz.
Experiments
1. Put a 0.1 mm wire into a laser beam and note the diffraction pattern.
Note the diffraction pattern when the laser is shone through a narrow slit.
Make the slit with 1. a laser printer, 2. a photocopier then print on clear plastic film, 3. a pin to draw a line on a piece of smoked glass from a candle flame.
2. Draw black spots on white paper, reduced them photographically, and use the positive and negative copies as complementary arrays.

27.2.1 Random multiple gratings interference
Experiment.
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.
Pass a collimated beam of white light through a glass dusted with Lycopodium powder.

27.2.4 Speckle spots and random diffraction interference
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.

27.7.9 Speckle patterns in arc light interference
Speckle patterns can also be seen in arc lamp light.
The patterns disappear as the object is brought closer to the arc.

27.7.8 Thin film interference, soap film interference
See diagram 27.7.8.0: Thin film interference.
See diagram 27.7.8: Soap Film Interference.
Thin film interference is caused by interference of reflected light from both the upper and lower surfaces of a thin film, e.g. an air / soap film boundary.
The thin films of soap bubbles and oil slicks produce swirling colour patters on their surfaces from constructive and destructive interference of light reflecting form the outer and inner surface of the thin films.
Soap bubbles are thin films suspended in air.
Oil slicks are thin films floating on water.
Some colours may be eliminated from incident white light by destructive interference.
Other colours may be enhanced by constructive interference.
The outer lens of binoculars may have an anti-reflective coating of a thin film to reduced reflection by destructive interference, so the lenses have a blue-purple colour.
Experiments
1. Turn on the light.
Dip the wire frame into the soap solution and secure with the clip.
Adjust the angle until the pattern appears on the side wall.
2. For 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.
3. For stable black soap films, use Vidal Sasson Extra Gentle Formula to make black films lasting five minutes or longer stable.
4. For a constant soap film, fit a large graduated cylinder with a rectangular frame with the handle protruding through the stopper and half fill it with soap solution.
5. Make a cup with a soap film by rotating a hemispherical shell with a soap film across the front so the black spot forms in the middle.

27.6.9 Newton's rings with float glass
See diagram 27.6.9: Newton's rings.
See diagram 27.5.3.2: Newton's rings with float glass.
The origin of Newton's rings is similar to the fringes formed by the air wedge, except an upper glass plate is replaced by a curved piece of glass with spherical cross section.
The circular fringes become more closely spaced moving away from the centre of the pattern, because the distance between the upper curved surface and lower flat surface increases faster with distance from the centre of the pattern.
A perfect lens can be tested, because the rings should be perfect circles.
Turn on the light.
The interference pattern is projected onto the side wall.
Change the pattern by tightening a screw on the edge of the glass disk.
2. Show interference of light by using float glass.
Float glass is produced by causing molten glass to spread over a bed of molten tin.
The stresses arise as the glass is allowed to cool and cut into slabs cause forms of curvature.
Use a slab of float glass illuminated by sodium light reflected from a white screen as in the diagram.
See the interference fringe patterns in the light reflected by the glass slabs.
See interference in transmitted light with the slabs in position AA'.
Note the complementary interference viewed in reflected and transmitted light.
3. 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.

27.6.15 Turpentine film
White light incident of the surface of turpentine on water at an angle of 45-60 degrees is focussed on a screen.

27.6.2 Absorption phase shift, monochromatic light
Cover the back of a microscope slide with streaks of an absorbing dye and observe under monochromatic light.

27.6.14 Tempering colours
A thin film of oxide forms on a polished steel sheet when it is heated.

27.6.10 Oil on water, petrol on water, thin film interference
Petrol and oil appears colourful when it contacts water.
They have lower density than water and do not mix with it so they sit on top forming a very thin film of petrol to spread out over the surface of the water.
A petrol film is so thin its thickness is almost the wavelength of the visible light spectrum.
Petrol is partially reflective leaving some light to be transmitted to the boundary with water which is also partially reflective.
The two reflected rays have different phases, so they may interfere destructively or constructively, or in between.
Total destructive interference is seen as a dark spot.
Total constructive interference is seen as bright spot radiating a colour with wavelength corresponding to the thickness of the film of petrol.
The film of petrol on water will appear colourful, the film does not have uniform thickness.
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.

27.7.6 Michelson interferometer
See diagram 27.7.6: Michelson interferometer.
Experiments
1. Turn on the laser.
Read the initial position of the micrometer.
Count the number of fringes while turning the micrometer slowly in one direction only.
Read the final position.
This can also be done with white light.
2. 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.

27.9.1 Dichroic polarization colours.
Dichroic lamps, jewellery, crystals, quinine iodosulfate, tourmaline.
Tourmaline, Al6B3Fe3H10NaO31Si6, (in Pegmatites), double refraction, contains about 10 % boron
Dichroism (Greek dikhros two colours) refers to substances that absorb light depending its direction or how much the substances are polarized.
So dichroic substances have a different colour when seen from different directions.
So dichroic jewellery has a reflected colour and a different transmitted colour.
As you look at it, the colours seem to change all the time.
Down lights, are usually halogen mirror reflector lamps, e.g. 12V 50W MR16 dichroic lamp, but these lamps are being ordered replaced by the Australian government, because of high energy usage.
Dichroic lamps have a coating on the reflectors of "multi-layer interference films" that selectively reflect or transmit certain wavelengths of visible light, IR, and UV to reduce the heat in the beam.
Dichroic crystals present different colours by transmitted light, when viewed in two different directions, the colours being unlike in the direction of unlike or unequal axes.
They absorbs the component of wave polarized in a particular direction.
Experiments
1. Hold a grid of parallel wires in a microwave beam and rotate the grid.
2. See quinine iodosulfate in viscous plastic where the crystals are oriented by extrusion, giant thin crystals of quinine iodosulfate.
3. Pass a polarized laser beam through a cylinder filled with a quinine sulfate solution.

27.3.5 Liquid cell absorption dispersion
Experiments
An absorbing solution is placed in a liquid cell placed in a beam of light before dispersion.

27.3.1 Band absorption spectrum, nitrous oxide, dispersion
Experiments
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.

27.3.3 Dispersion curve of a prism
Experiments
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.

27.3.2 Deviation with no dispersion
Experiments
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.

27.3.4 Dispersion of fuchsin and sodium, anomalous dispersions
Experiments
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.

27.3.6 Scattering, Rayleigh scattering, Mie scattering
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.
Experiments
1. Red and blue beam.
Pass a red beam is passed through a solution of gum mastic, but a blue beam does not pass through it.
2. Colour of smoke.
Cigarette smoke is blue. but after exhaling is white.
3. Note 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.

27.107 Primary and secondary colours
See diagram 27.102: Mix primary colours.
The primary colours, red, green and blue, cannot be produced from other colours of light, but they can be mixed by shining individual beams on a white screen so that they overlap to produce secondary colours.
Red light + blue light gives secondary colour magenta.
Blue light + green light gives secondary colour cyan (turquoise, peacock blue).
Green light + red light gives secondary colour yellow.
The three primary colours or the three secondary colour together give white light.
Complimentary colours are colours that combine to form white light.
Blue light + yellow light gives white light.
Green light + magenta light gives white light.
Red light + cyan light gives white light.
Coloured lights are mixed by addition.

27.108 Coloured Pigments
Colourex Pigments sorted by colour, (Commercial).
Subtractive colour effect
Pigments in paints and dyes reflect light of certain colours and absorb all the other colours.
The primary colours of pigments, called subtractive primaries, are magenta, yellow, and cyan.
When white light shines on coloured paint, only some of the wavelengths of light are reflected.
Cyan paint absorbs red light. but reflects blue and green light.
Yellow paint absorbs blue light. but reflects red and green light.
If cyan paint is mixed with yellow paint, you see green paint, because both red and blue light are absorbed and only green light is reflected.
Coloured pigments are mixed by subtraction.

27.124 Dichromatism
Resazurin, C12H7NO4, is dark blue when first added to the solution to be tested then becomes pale blue then becomes pink or purple on shaking.
Resazurin has dichromatism (polychromatism), where the hue of the colour depends both on the concentration of the absorbing substance and the thickness of the medium the light passes through.
Other dichromatic substances are bromophenol blue.
Dichromatism is observed if the absorption spectrum of any substance has at least two local minima, one wide but shallow and one narrow but deep local minimum.

27.180 Rainbows
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, 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 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.

27.6.16 Wave front models
See diagram 2.0.5: Conic sections, ellipse.
Experiments
Wire models show spherical and elliptical wave fronts in crystals.

27.190 Sunset with polarizer
Experiments
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.

27.9.3 Polarization by scattering
Experiments
1. Show that scattered light is polarized.
When the direction of observation is normal to the primary beam, the light is completely linearly polarized.
2. Add milk to water and show polarization of light scattered from a beam.
Use a slide projector is used to illuminate the solution.
The solution appears bluish white in scattered light and yellowish in direct light indicating that Rayleigh scattering predominates.
3. Use a large sheet of polaroid to investigate the polarization of the scattered light.
4. Hold a grid of parallel wires in a microwave beam and rotate the grid.
Rotate a slotted disc in a microwave beam.
5. A "hamburger grill" filter is used to demonstrate polarization from a 12 cm dipole.
6. Construct a strip grating that can convert a linearly polarized plane wave into one that is circularly polarized.
7. Two boxes, one a polarizer and the other an analyser, are built with a centre slot that can be oriented either horizontally or vertically.
Use with waves on a rubber hose.
8. Two large wooden slits oriented parallel or perpendicular to one another with a long helical spring passing through both.
9. 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.

27.9.2 Haidinger's brush
In 1846, Wilhelm von Haidinger, Austria, perceived a faint yellowish stain or "brush" that remained when he looked directly at light.
The "brush" rotated together with the rotation of a polarizer, so he was observing polarization.
Experiments
Train yourself to detect polarized light with the naked eye.
Look through a sheet polarizer at a piece of white paper illuminated by the sun or look at a white cloud for, 20 seconds, rotate the polarizer 90o, look again for 20 seconds, continue rotating back and forth every 5 seconds.

27.4.8 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.

27.194 Spectroscope for materials analysis, shoe box spectroscope
Spectroscopes (Commercial).
See diagram 36.101: Shoe box spectroscope.
By using a sensitive instrument called a spectroscope, scientists are often able to analyse the composition of materials located a great distance away.
The spectroscope has been used to determine the composition of the Sun and other stars and of the atmosphere of many of the planets.
Spacemen in the future will use this kind of device to analyse the chemical composition of their immediate surroundings.
Light entering a spectroscope is split up by a diffraction grating to form coloured bands, a spectrum.
Since each chemical element shows certain characteristic bright shoe box spectroscope lines in its spectrum the material can thus be easily identified.
Experiments
The materials required are a shoe box, replica grating, see science supply catalogues, some masking tape, and a double edged razor blade broken in two.
Cut a hole of about 2 cm diameter in the middle of one end of the box.
Use tape to fix a piece of replica grating over the hole from the inside.
Cut a 2.5 cm × 0.5 cm slit, which should be parallel to the lines of the grating, in the middle of the other end.
Cover the slit from the outside with a finer slit made from two halves of a razor blade, edges facing each other.
The two halves are held together and fixed to the box with tape.
The width of the slit should be about the same as the thickness of a razor blade and is finally adjusted for the best results, see diagram.
Look through the spectroscope at various luminous gases such as neon and argon in lamps or signs.
Notice the bright lines in the spectrum, which indicate that each element has its own pattern.

27.195 Atomic Absorption Spectroscopy, (AAS)
Atomic absorption spectroscopy, AAS, is used to determine the composition of inorganic elements in a sample.
AAS works on the principles of atomizing a sample and quantitatively determining the concentration of atoms in the gas phase by measuring the intensity of light absorbed by them when they are irradiated with electromagnetic radiation.
A key aspect of AAS is the method used to atomize the sample, which also affects the sensitivity of the technique.
Methods include flame, graphite furnace, electro-thermal, plasma furnace, and vapour-hydride atomization.
Chemicals prepared for AAS work are called AAS solutions.
Spectroscopy does destroy samples, but the interfering parts must be removed.
In AAS analysis for trace metals in blood, biopsy or plant material, the sample tissue must first be digested, usually with perchloric acid or nitric acid.

27.8.0 Magenta
Magenta cannot be emitted as a wavelength of light.
Magenta is the complementary colour to green, i.e. afterimage seen after staring at a green light.
All the colours of light, except green, have a complementary colour in the visible spectrum.
The brain is said to "average" wavelengths of light seen record a colour.
Mixed red light and green light is seen as yellow light.
However, mixed violet light and red light is seen as magenta, not the "average" wavelength, green.

27.1.0 CERN school experiments
CERN (Conseil Européen pour la Recherche Nucléaire)
1. Cloud Chamber, Build and Observe a Particle Detector
Students build their own particle detector using dry ice and isopropanol to make cosmic particles and natural radiation visible.
Students study the properties of the different tracks and discuss their observations.
Safety: Students must follow the safety rules and use the appropriate safety equipment provided.
This experiment involves working with dry ice (approx. -80°C), isopropanol at >90% concentration, and working with torches in the dark.
2. Electron Tube, The Basics of Particle Acceleration
Students operate an electron beam (300 eV) and study electrons in magnetic fields.
Safety: Power supply (300 V) will only be connected using safety leads, vacuum tube needs to be handled with great care.
3. X-Rays, Medical Applications and Pixel Detectors
Students operate an X-ray machine (35 keV) and study the absorption of X-rays in matter using a fluorescent screen and a pixel detector.
Safety: The X-ray units are fully protected devices and specially designed for use in schools.
They present no danger from ionising radiation.
4. Positron-Emission-Tomography (PET), Medical Applications
Students calibrate and use scintillation detectors to understand the basic principles of Positron-Emission-Tomography (PET) and locate a positron source (Na-22).
Safety: Radioactive sources (74 kBq Na-22) will only be handled by the supervising team.
Pregnant women and students younger than the minimum age specified are not allowed to take part in this experiment.
The power supply (700 V) will only be connected using safety leads.
Students must handle the fragile and expensive equipment with great care.
5. Superconductivity, Resistance is Futile
Students measure the electrical resistance of a normal conductor and a high-temperature superconductor (Bi-2223).
Students study the Meissner-Ochsenfeld-Effect and the Flux-Pinning-Effect.
Safety: The liquid nitrogen used in the experiments has a boiling point of approx. -196°C (77K).
Liquid nitrogen will be distributed only by the supervising team.
Some of the magnets used are very strong and must handled with care by the students.

Table 27.4.5 Types of radiation in the electromagnetic spectrum

Type of. radiation	γ-rays	X-rays	Ultraviolet. rays	Visible. light rays.	Infrared. rays	Microwaves	Radio / TV waves
Wavelength, λ (m). .	< 1 x 10-11. .	1 x 10-11 -. 1 x 10-8.	1 x 10-8 -. 4 x 10-7.	4 x 10-7-. 7 x 10-7.	7 x 10-7-. 1 x 10-3.	1 x 10-3-. 1 x 10-1.	> 1 x 10-1. .
Frequency, f, (Hz).	> 3 x 1019. .	3 x 1016-. 3 x 1019.	7.5 x 1014-. 3 x 1016.	4 x 1014-. 7.5 x 1014.	3 x 1011-. 4 x 1014.	3 x 109-. 3 x 1011.	< 3 x 109. .
Energy, (J).	> 2 x 10-14. .	2 x 10-17-. 2 x 10-14.	5 x 10-19-. 2 x 10-17.	3 x 10-19-. 5 x 10-19.	2 x 10-22-. 3 x 10-19.	2 x 10-24-. 2 x 10-22.	< 2 x 10-24. .

nm, nanometres, 1 nm = 10-9 m.
Å, ångström, 1 Å = 10-10 m.