Electromagnetic Radiation Main Index

Electromagnetic Radiation


Electromagnetic radiation (emr) is produced in many ways, and there are many different types.

Long, medium and short waves are usually known collectively as radio waves.

All electromagnetic radiation travels at the speed of light (about 300 × 106 m/s in a vacuum), and the relationship between the wavelength, the frequency and the speed of propagation is given by

c = lambda × f

where c is the speed of light in metres per second, lambda (the Greek letter lambda) is the wavelength in metres and f the frequency in hertz (Hz), after the German physicist Heinrich Hertz (1857 - 1898).

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.

For VHF radio waves we usually use the frequency, for example BBC Radio 1 is in the range 97 to 99 megahertz (depending on where you live in the UK), but for longer and shorter waves we usually use the wavelength, for example if you live in Aberdeen BBC Radio 4 is on 1449 metres, whereas red light is about 700 nanometres - wherever you live! But you can always convert from one to the other using the formula above on this Page, for example a wave with a frequency of 97 MHz has a wavelength of about 3 m.

Like all other sorts of waves, emr can be reflected, refracted, transmitted, diffracted and absorbed - the way emr behaves depends upon its wavelength.

Long waves

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. This reflection of long waves by the ionosphere allows LW radio transmissions to be received anywhere on the Earth, hundreds or even thousands of kilometres away from the transmitter, but it also means that reception of LW is subject to interference from other LW radio stations, anywhere in the world.

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Medium and short waves

Shorter wavelengths (MW, SW, VHF, UHF and SHF, for medium and short wave and very, ultra-high and super-high frequencies) are reflected much less by the ionosphere, the shorter the wavelength the less they are reflected, so they do not travel so far round the Earth. This makes VHF very suitable for interference-free local radio and tv transmissions. Transmissions to and from spacecraft and artificial satellites (including of course television satellites) must not be reflected by the ionosphere at all so are SHF.

Radar waves must have wavelengths of less than the size of the object they are being reflected off: the use of aircraft carrying 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.

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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 - this includes your very best china plates, the ones with the gold-leaf decoration! 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.

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Infra-red waves

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.

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Light waves

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.

There is more about light and colour and colour blindness on the Colour Vision Page.

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Ultra-violet rays

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.

Even low levels of UV light produce vitamin D in our skin, and if you are getting very little sunlight, for example never going outside in the winter (or living inside the Arctic Circle!), foods containing lots of vitamin D may help keep you healthy, but higher levels, for example strong sunlight in the summer months, can also produce sunburn and other skin damage, 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. “A nice tan” is one thing, skin cancer something very different.

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 - you can read about this on the Atmospheric Pollution Page.

Ultra-violet radiation is emitted by very hot bodies although it is absorbed by glass so the light from old-fashioned light bulbs (the sort with a thin white-hot wire inside) does not contain uv light. In science lessons you must never watch burning magnesium except through a piece of special blue glass.

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 substances (phosphors) which fluoresce. 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. Very cheap fluorescent tubes (ordinary strip lights) have only one or two of these 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. Today many low-energy lamps are CFLs (compact fluorescent lamps).

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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 so that it shows up on the X-ray.

X-rays can cause cell damage, particularly to babies and children, 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.

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Gamma rays

gamma (the Greek letter 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 gamma 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.

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© Barry Gray May 2016

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