Somewhere Over the Rainbow: The Discovery of Infrared Light

Hello, friends!

The way I see it, there are two kinds of people who call themselves skeptics.  There are skeptics who question everything because they genuinely want to learn more, and then there are skeptics who question everything that does not conform to their own particular worldview.

I was once sitting in a bar with a young woman who turned out to be that second type of skeptic.  The conversation turned to outer space (as conversations inevitably do when I’m around), and this young woman kept asking: “How could they possibly know that?”  And when I said I honestly didn’t know, she concluded: “I think scientists just make all this stuff up!”

So today, I’d like to start what I’m hoping will become a series of posts on this blog answering the question: “How could they possibly know that?”  And we’ll begin with the discovery of infrared light.


You may be surprised to learn that infrared light was discovered in the year 1800.  Sir William Herschel (the same Sir William Herschel who’d previously discovered the planet Uranus) was tinkering with his telescope, trying to find a safer way to observe the Sun.  He thought that, perhaps, different colored filters might do the trick.

So Herschel set up an experiment to measure the temperatures of different colors of light.  It was an elegantly simple experiment.  A ray of sunlight passed through a prism, and the rainbow of light that came out of the prism hit some thermometers.

Herschel found that the blue/violet side of the spectrum was associated with lower temperatures; the red/orange side was associated with higher temperatures.  This was not, actually, what Herschel had expected.  He’d thought temperatures would peak somewhere in the middle: in the yellow/green part of the spectrum.

Curious, Herschel decided to place a thermometer outside the visible spectrum, somewhere beyond red.  The dark area beyond red turned out to be hotter than any of the visible colors.

Herschel called this new, invisible kind of light “calorific rays,” from a Latin word meaning “heat.”  The word calorie comes from the same Latin root.  The term infrared light would not be introduced until many decades after Herschel’s death.


Of course you do, because you’re the first kind of skeptic I mentioned, not the second!  Here are some links, organized from “easiest and most accessible” at the top to “most technical” at the bottom.  Enjoy!

P.S.: The word infrared literally means “under red.”  So this blog post really should have been titled “Somewhere Under the Rainbow.”