Electromagnetic radiation A form of energy that travels in waves at the speed of light (in a vacuum). Electromagnetic waves differ from sound waves, which need a medium, like air, water or stone, to travel through. Radio waves, microwaves, light, radiant heat and X-rays are all forms of electromagnetic radiation. They all have different wavelengths—the distance between one wave energy-peak and the next—and therefore different frequencies—the number of waves per second. Different forms of electromagnetic radiation are grouped in the electromagnetic spectrum according to their wavelengths. Those with shorter wavelengths carry the most energy. Visible light is the only part of the spectrum that we can see.

Gamma rays A form of electromagnetic radiation with a very high frequency and short wavelength. They are given out by radioactive atoms or produced in nuclear reactions. They can pass right through some substances.

Infrared (IR) radiation A form of electromagnetic radiation, with a wavelength just longer than that of visible red light. Some IR can be felt as heat from hot objects. IR radiation is used for short-range communications, such as in television remote controls and remote locking in cars.

Maxwell, James Clerk (1831–1879) Scottish physicist who predicted the existence and speed of electromagnetic waves.

Microwaves A form of electromagnetic radiation with a wavelength longer than infrared radiation. Microwaves are used in communications. Some microwave frequencies can also be used in cooking.

Photons Light and other forms of radiant energy behave as if they are waves, but in fact they are streams of tiny particles, called photons. Photons have no mass or electrical charge. In a vacuum they travel at the speed of light.

Radar (Radio Detection And Ranging) A system that detects objects and measured their speeds by transmitting radio waves and receiving the “echoes” that bounce back from them. Radar is used by ships and aeroplanes to detect other craft or oncoming storms.

Radio waves A form of electromagnetic radiation with a very long wavelength. Radio waves are important in communications through air and space.

Röntgen, Wilhelm (1845–1923) German physicist who discovered X-rays.

Transmitter An electronic device that sends out radio waves.

Ultraviolet (UV) radiation A form of electromagnetic radiation with a wavelength just shorter than that of visible violet light. Some of the energy emitted by the Sun takes the form of UV radiation. Most UV rays from the Sun are absorbed by the Earth’s atmosphere, but enough to reach Earth and cause tanning of the skin and sunburn.

Visible light The part of the electromagnetic spectrum that is visible to the human eye. Light waves with slightly different wavelengths appear as different colours. The colour red has the longest wavelength and violet has the shortest wavelength.

X-rays A form of electromagnetic radiation with very short wavelengths and extremely high frequencies. They pass right through some solids. In medicine they can be used to see the structure of bones and internal organs by placing a patient between an X-ray source and a photographic film or camera.

Consultant: Mike Goldsmith