Infrared is a whole spectrum of light that we can only see in Predator movies, and special episodes of Cops. How did people first discover this stealthy light? It all started with a bunch of thermometers in the hands of an astronomy genius — and it's an experiment you can reproduce at home.
Frederick William Herschel was most famous for discovering Uranus. (I'll give you time to recover.) He also discovered something less tangible. (And again.) This was the guy who realized that light came in more than just the varieties that we can see. He managed to do this with relatively little equipment, using an easy test.
The entire experiment started because Herschel wanted to measure the temperatures of the different colors of light. He grabbed a prism, and set it out on a sunny window. The prism did what they always do — split the incoming white light up into different colored stripes, to form a rainbow. Herschel placed thermometers in each of the stripes of that rainbow. The bulbs of the thermometers were painted black which, because of its absorption of all light, is better able to suck up all the available heat. He found that the red light was hotter than blue light. He then placed a thermometer bulb just beyond the reach of the light, and watched the temperature shoot up even higher.
With a little thought, and a few more repetitions of the experiment, Herschel realized that there had to be light streaming down on the thermometer bulb, that appeared to be out of reach of the rainbow. It was just light that people couldn't see. Since this light seemed to hang out at the red end of the spectrum, he called it infrared. If you isolate a prism and a few thermometers with black-painted bulbs, you can re-do his experiment any time you want!
Top Image: Zatonyi Sandor