Discovery of Infra-red light

Great experiments in physics that students can do:

II. Bring One of the Great Moments of Science to the Classroom

Herschel's discovery of infra-red light

     In 1781, Frederick William Herschel, an astronomer, discovered the planet Uranus, out beyond Jupiter and Saturn.
     A scientist of wide-ranging interest, he later became intrigued by a well known but not yet understood phenomenon: the separation of star (and sun) light into the rainbow colors when it is passed through a prism.
     Knowing that light from the sun produces heat when it hits objects on earth, he wondered whether each of the separate colors could do this alone, or only as part of the whole.
     He arranged a prism so that the rainbow colors were spread far enough apart so that he could put a thermomenter in the area of each color. (He knew enough to blacken the bulbs of the thermometers to better absorb heat; if you do this experiment you will have to take care to do the same.)
     Being a good scientist, he used one additional thermometer as a "control." He placed that thermometer outside the rainbow altogether, where no light was visible at all. He expected the control thermometer in the unlighted area to show no temperature rise. He was surprised to find that the control thermometer placed beyond the red edge of the rainbow, registered a greater temperature rise (in equal time) than any of his thermometers in the various portions of the visible rainbow!
     After a great deal of further study, he concluded that there was light there, beyond the red part of the rainbow, and that this light produced heat, although it was not visible to us. The biochemistry of vision was not yet known, and so he did not understand how this light could produce heat like the visible colors, but could not be seen by humans.
     This light, because it was beyond the red part of the rainbow, was called infra-red light.
     Nowadays, although our eyes can not see this infra-red light, we can make "artificial eyes" that are sensitive to this light. In cameras, these infra-red sensing devices can even make pictures using this light.
     So-called "night photography" makes images on film using this light. In total darkness, your body warmth gives off infra-red "light", more so than the colder objects around you, so that you are plainly visible to infra-red cameras, even if your friends can not see you.

     Hershel's experiment is so simple that it is one that you can do. All you need is a good prism (one whose glass transmits some infra-red light) and several thermometers.

     It is important that you do the experiment outdoors or with wide-spectrum indoor lights, because much of our indoor light has its infra-red light trimmed off by the glass bulbs through which the light passes.
     It is essential that the bulbs of the thermometers are painted black, and that you have a certain amount of patience. It may take some time for the thermometers to absorb enough heat to raise their temperatures measurably.

     NASA and the Jet Propulsion Laboratory, agencies which do extensive work in scanning the skies for infrared signals, have put together a more detailed set of instructions to do this experiment as an educational service.
[Note: These instructions were recently available on the internet. They are no longer at the site we quoted. We are inquiring whether they are available elsewhere on the internet, or by request. In the meanwhile, we have removed the non-working link.]
     Good luck

Permission is hereby granted to reproduce the contents of this section for use in teaching, provided no charge or fee is accepted and provided credit is given to Cavendish Science Organization

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