Electromagnetic radiation (emr) is produced in many ways. All electromagnetic radiation travels at the speed of light (300 × 106 m/s in a vacuum), and the relationship between the wavelength, the frequency and the speed of propagation is given by
× f
where c is the speed of light in metres per second, (lambda) is the wavelength in metres and f the frequency in hertz.
The wavelength of emr ranges from more than a kilometre for some radio waves to less than a picometre (a million millionth of a metre, 10-12 m) for some gamma rays.
Like all other sorts of waves, emr can be reflected, refracted, transmitted, diffracted and absorbed - the way emr behaves depends upon its wavelength.
Long wave (LW) radio transmissions, with wavelengths of up to or more than a kilometre, are reflected by the ionosphere, a layer of charged particles in the Earth’s atmosphere at a height between 80 km and 400 km. The ionosphere used to be called the Heavyside Layer after the man who discovered it. This reflection of long waves allows LW radio transmissions to be received over distances of hundreds or even thousands of kilometres. This means of course that LW transmissions are very subject to interference from other LW radio stations anywhere in the world. Shorter wavelengths, measured in metres or centimetres, ( MW, SW, VHF, UHF and SHF, for medium and short wave and very, ultra and super-high frequencies) are not reflected by the ionosphere so cannot be received over very long distances, although they do bend round the Earth's surface to some degree, so the transmitter and receiver do not have to be in line of sight. This makes VHF very suitable for interference-free local radio and tv transmissions. Transmissions to and from spacecraft and artificial satellites (including of course satellite television) must be able to penetrate the ionosphere so are SHF. Radar waves must have wavelengths of less than the size of the object they are being reflected off: the development of centimetric radar (using radio waves with wavelengths of a few centimetres which could be reflected off the periscopes of submerged submarines) was a major factor in winning the Battle of the Atlantic against German U-boats in the Second World War.
Microwaves have wavelengths of a few millimetres (10-3 m). They are used by mobile phone networks and microwave ovens. The microwaves used in a microwave oven have a wavelength which allows them to be absorbed by water molecules. This means that they only heat substances (such as almost all foods) containing water. They pass through glass and plastic but are reflected by metals - it is usually very dangerous to put any metal object in a microwave oven. As they only heat substances containing water they do not heat the dish containing the food (except by transmission from the hot food) so food heated on a china or glass plate in a microwave oven cools down very quickly once microwave heating stops.
Infra-red waves (beyond the red) have wavelengths of a few µm (micrometers, 10-6 m). They are emitted by hot objects and carry radiant heat, including of course the radiant heat from the Sun. Warm-blooded animals, including Man, give off infra-red radiation and this effect is used in many ways, from burglar alarms to night-sights on rifles.
Light waves have wavelengths from about 700 nm (nanometres, 10-9 m) for red light to about 400 nm for violet light. The longer wavelengths penetrate mist and fog better than shorter ones, which is why red lights have been used for danger for hundreds of years. Green lights for safety were only introduced in the 1840s for use on the railways. This was an unfortunate choice of colour because in Europe about one man in ten is colour-blind and has difficulty in telling the difference between red and green: that is why today “green” traffic lights are actually quite blue.
Wavelengths of less than that of violet light cannot be seen by Man although many animals, including bees, can see them. They are called ultra-violet (UV) light, and can cause changes in cells. They cause suntan or sunburn, and it is important that people, but particularly children and those with fair skins, protect themselves from prolonged exposure to high levels of UV light. UV light is often divided into two bands: UVA has wavelenghts in the range 380 - 320 nm and UVB in the range 320 to 280 nm. UVB is much more harmful than UVA.
Most of the UV light from the Sun, and much other harmful radiation from space, is absorbed by the ozone layer high in the Earth's atmosphere. This is why measures to stop the destruction of the ozone layer are so important - to read about this please click here 
Ultra-violet radiation is emitted by very hot bodies although it is absorbed by glass so the light from ordinary light bulbs (the sort with a white-hot thin wire inside) does not contain uv light.
Many substances fluoresce, that is, give off visible light when subject to ultra-violet light. The gas in a fluorescent light tube gives off ultra-violet light when an electric current passes through it; the inside of the glass is coated with a substance which fluoresces. The visible light produced passes through the glass of the tube but the ultra-violet light is absorbed by it and so does not pass through it into the room. Cheap fluorescent tubes (strip lights) have only one or two of these chemicals (phosphors) so give out light of only one or two wavelengths and a very unnatural type of light; more expensive ones contain many more phosphors and so give out a much more pleasant and natural light.
X-rays have wavelengths of a few nanometers. They are absorbed by certain types of atom, including calcium and barium, so they can penetrate most soft tissues but not bone, which is made of calcium phosphate, hence their use in medicine. The stomach and intestine do not usually show up on X-rays, so if you need a stomach X-ray you have to drink some barium sulphate just before the X-ray. X-rays can cause cell damage, particularly to bone cells, which is why pregnant women are not usually given X-rays. They are emitted when a beam of electrons is fired at a metal target.
(gamma) rays have wavelengths of a few picometres (10-12 m). They are emitted by atomic nuclei and are very penetrating. They can do a lot of damage to living cells. X-rays and rays have wavelengths of the same order of magnitude as the spaces between the atoms in many crystals, and so can be diffracted by crystals. This makes them very important to scientists studying the structure of crystals and molecules.
© Barry Gray February 2007
