Many folks know about prisms. A prism is a triangular piece of glass that breaks white light up into its constituent colors. That is, shine white light in one side of a prism and a rainbow will come out another side. All those colors were originally in the white light, but mixed together. Now they are separated out: red, orange, yellow, green, blue, indigo, violet.
Now here's a question for you. Do all of those bands of color have the same temperature?
Hmmm, you say. Why wouldn't they?
Well, they might not for the same reason that they don't have the same color. What we see as color is an indication of the frequency of the light wave that is hitting our eyes. A relatively low-frequency light wave will look reddish to us, while a higher-frequency wave will appear bluer.
In 1800 an astronomer named William Herschel was interested in just this question. He wanted to see if different colors coming out of a prism have different temperatures. His experiment was simple: he held a thermometer in each band of light and recorded the readings. However, something unexpected happened.
Much to his surprise, Herschel found the thermometer gave a sudden jump in its reading when he held it just past the reddest band. Where the thermometer was sitting Herschel saw no color at all, but the temperature there was the highest! Can you guess why?
It was a great moment in physics. Herschel had discovered a wavelength of radiation with a lower frequency than we can see, but which exists nonetheless. We now call his finding infrared radiation.