A form of energy that propagates through a vacuum at a speed c of 3 × 10 to the power of 8 metres per second. The name reflects the nature of the radiation, which consists of linked and rapidly varying electric and magnetic fields. Its character varies according to wavelength λ. Radio waves have the longest wavelengths, ranging from several metres to millimetres. The shortest radio waves are usually termed microwaves. These merge into the infrared, which ranges down to just under a micrometre. Visible light is a narrow band of wavelengths between about 700 and 400 nanometres nm. Ultraviolet goes down to about 10 nm, then X-rays to 0.1 nm. The shortest waves of all are gamma-rays. The full electromagnetic spectrum is the whole range of radiation types, from the shortest wavelengths to the longest. The Ã¥ngström is also used as a unit of length for the measurement of wavelength. 1 Ã… = 0.1 nm. Electromagnetic radiation, in common with any wave, also has an associated frequency ν. The link between frequency and wavelength is ν = c/λ. Thus, as wavelength decreases, frequency increases. The energy E associated with electromagnetic radiation increases in direct proportion to frequency, according to the relation E = hν, where h is Planck’s constant. The energy is quantized in units of this size, which are termed photons. Electromagnetic radiation and its detection are crucial in the study of astronomy, which depends almost entirely on the receipt and analysis of such radiation from distant objects. Optical and radio astronomy can be undertaken from the ground because these wavelength bands pass relatively unobstructed through the atmosphere. Astronomical observations in other wavebands are largely carried out from orbiting spacecraft, though some are possible from high mountain sites and aircraft.