When an object is heated it radiates energy in the form of electromagnetic waves. These waves go from the radio wave end of the electromagnetic spectrum through the infrared, the visible, the UV, X-ray and gamma ray. Ultraviolet is invisible to human beings, but unfortunately, in significant amounts is extremely harmful. Infrared is also invisible and produces a sensation of heat, but fortunately with no adverse side effects. For most of the hot objects that we encounter on the planet earth the energy lies mainly in the infrared region. A cooker hot plate may show no visible signs of heat, but when a hand is held over it, it feels warm; we are experiencing infrared at about 500 K. When the hot plate is turned up and reaches 1000 K we see a slight glow coming from the element which is the onset of visible radiation. The plate also becomes noticeably hotter, showing
that there is an increase in the amount of radiated energy. A good example of infrared radiation is that from the black luminaires that are used in the entertainment industry. There is no visible sign of heat from the body of the luminaire, although we get a nasty burn if we touch it.