William Herschel and the discovery of infra-red radiation
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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