EMISSION OF THERMAL LIGHT

We have all undoubtedly encountered thermal light in the form of emission of light from a red-hot object such as an element on an electric stove. Other examples of such light are in the common incandescent electric lamp, in which electrical current flowing through a thin filament of tungsten metal heats it until it glows white-hot. The energy is supplied by an electrical current in what is called resistance heating, but it could just as well have been supplied by, say, a gas flame. In fact, the original incandescent lamp was developed in 1825 for use in surveying Ireland and was later used in lighthouses. The lamp worked by spraying a mixture of oxygen and alcohol (which burns incredibly hot) at a small piece of lime and igniting it. The lime was placed at the hottest part of the flame and heated until it glowed white-hot, emitting an immense quantity of light. It was the brightest form of artificial illumination at the time and was claimed to be 83 times as bright as conventional gas lights of the time. Improvements to the lamp were made by using a parabolic reflector behind the piece of lime concentrating the light. The lamp allowed the surveying of two mountain peaks over 66 miles apart and was later improved by using hydrogen and oxygen as fuel but eventually was superseded by the more convenient electric arc lamp. This light source did, however, find its way into theaters, where it was used as a

Figure 1.1 Electromagnetic spectrum.

spotlight which replaced the particularly dangerous open gas flames used at the time for illumination, and hence the term limelight was born.