Electromagnetic radiation

Electromagnetic radiation is the general name given to a whole family of waves ranging from gamma rays to radio waves.
(See: Electromagnetic spectrum)

The main types of electromagnetic waves are gamma rays, X rays, ultra violet, light, infrared, microwaves, TV and radio.


The properties of these waves depend on where they are in the "Electromagnetic spectrum" but they all have certain things in common.



(a) All electromagnetic waves are transverse waves which means the waves vibrate at right angles to the way in which they are moving
(b) They all travel at the same speed in a vacuum (300000 km/s). We usually call this speed the 'speed of light'.
(c) Unlike sound waves they can all travel through a vacuum
(d) They slow down when they move through different materials. For example light moves slower in glass then it does in a vacuum and a radio signal moves slower in a wire than it does when travelling though a vacuum (or air) towards the receiving aerial.

The very high speed of electromagnetic waves means that a radar pulse only takes 2.5 seconds to travel to the Moon and back but the enormous distance in the Universe means that light from a distant galaxy like the one shown in the photograph still takes over thirty million years to reach us!

As we have already said the properties of a particular radiation depends on where it is in the electromagnetic spectrum, - this means on its wavelength and frequency. Gamma radiation has a very small wavelength but a very high frequency while radio waves have a much lower frequency but a far bigger wavelength.

Electromagnetic radiation is emitted a small packets of energy called photons. These photons are emitted when electrons fall from one energy level to another inside an atom. (See: 14-16/Atomic physics/Text/Emission of light)

The more intense the source of radiation the more of these quanta are emitted per second. The energy of each quantum is closely related to the type of radiation.

Radiation with a high frequency is emitted as photons of high energy, it can be dangerous to humans and is very penetrating – for example gamma radiation can pass through many centimetres of lead. On the other hand the photons of radio waves are of much lower energy and if you put a radio in an aluminium box the radio waves could not penetrate the aluminium and you would not receive a signal.