Discovery of infrared. Historical artwork showing the experiment in which infrared light was discovered in 1800. Sunlight enters at top left and the colours that form it are refracted (bent) in the prism by different amounts. The separated colours form a colour spectrum on the table. Using thermometers (centre right) the temperature at various parts of the spectrum was found. However, the hottest area was discovered to be beyond the visible red light (where the lowest thermometer is seen). This infrared light is part of the electromagnetic spectrum which also includes visible light. The experiment was done by the astronomer Frederick William Hershel (1738-1822).