http://www.practicalphysics.org/go/Resources_16.html In 1800, William Herschel published a series of papers describing experiments which led him to identify infra-red radiation, a form of heat radiation beyond the red end of the spectrum of visible light. Background to the experiment When visible light falls on a surface, some of it may be absorbed; the surface is warmed. A thermometer left lying in sunlight shows an increasing temperature. Today, we would say that light transfers energy from the Sun to the Earth, and when it is absorbed, the energy becomes internal energy (‘heat ’) in the absorbing material. We also receive infra-red radiation and ultraviolet radiation from the Sun. However, the nature of these was unknown in 1800. William Herschel and his sister Caroline Herschel were astronomers working in Bath. William had been using telescopes to observe the Sun. Of course, this is hazardous, so he used dark glass filters to reduce the intensity of the Sun ’s rays. He noticed that he could still feel the heat of the Sun’s rays coming through the filters. He also noticed that some filters seemed to allow through more of the light, while others transmitted more of the heat. He wrote that, when observing the Sun, he used: ...various combinations of differently-coloured darkening glasses. What appeared remarkable was that when I used some of them, I felt a sensation of heat, though I had but little light; while others gave me much light, with scarce any sensation of heat. Herschel set about comparing how well these different glass filters transmitted heat. He needed to know whether his observations were being affected by the filters he was using. --

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