All bodies radiate energy on the electromagnetic spectrum. As the temperature of the body increases, the wavelength of the radiation changes with its peak occurring at lower and lower wavelengths. Bodies at typical temperatures on Earth radiate with a peak in the infra-red spectrum, but as you heat up a metal in a furnace it glows “red hot” then “white hot” because it is starting to emit radiation of shorter wavelength than infra red - radiation in the visible spectrum.

The surface of the Sun is at a temperature of around 5,500°C, and the diagram on the left shows its emission spectrum – the energy emitted at each wavelength of radiation. The peak is in the visible spectrum, with components in the Ultraviolet and Infra-Red range.

Gas molecules such as water and oxygen in the Earth’s atmosphere absorb certain wavelengths, causing so-called knock-out bands. The energy reaching sea level is shown in blue.