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.
HOW DO THEY KNOW THAT?
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.
WANT TO LEARN MORE?
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!
- A historical reenactment of Herschel’s experiment on YouTube.
- “Herschel’s Experiment” from Cool Cosmos.
- “Herschel and the Puzzle of Infrared” from American Scientist.
P.S.: The word infrared literally means “under red.” So this blog post really should have been titled “Somewhere Under the Rainbow.”
Planet Pailly 
Nice placement of links at the end 🙂 It doesn’t interrupt my reading flow and looks good. No one could explain every discovery to a friend at a bar. Especially not if you have to trace back through decades of data and analysis. Do you ever check out the podcast and transcripts from Skeptoid? That’s a site I like. https://skeptoid.com/episodes/4530 for for why you shouldn’t question everything. That statement should pique some interest among this blog’s readers.
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That certainly piques my interest! Thanks!
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Under red – makes sense. And red has always been associated with heat, so it really shouldn’t have been so surprising that was the hottest.
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I think Herschel was picturing something like a bell curve, with the highest temperature in the middle. I can kind of imagine why he would expect that.
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I love the idea of the “how could they know that” series. Looking forward to future entries!
I wish a had money for every time I’ve been told I’m not a skeptic by someone because I didn’t share their biases. Granted, we’re all Bayesians to some extent, but it matters how our priors were arrived at.
Kind of interesting that the longer wavelength is the hotter one. Given that the shorter wavelengths (like ultraviolet) are supposed to have more energy, I wonder how that works.
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You know, I’ve been wondering about that too. I’ve wondered about that for a long time now, but I never looked into it.
After writing this post, though, I started reading a little about the discovery of ultraviolet light. If I do more research on that, I have a feeling I might find the answer to our question.
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I didn’t know about that experiment.. that’s cool!
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Isn’t it? I was shocked that somebody could figure that out all the way back in 1800!
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I know – All the cool stuff was worked out so far back…
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That’s really interesting, and I wouldn’t have thought the reds are hotter than the blues! I’m curious as to why, but I have a feeling that since the intensity and amount of infrared is much higher than purple, there’s more light transferring more energy in that part of the spectrum. I can’t find much to explain his findings though.
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Yeah, I’m a little confused about that myself. It’s the opposite of how black body radiation works. I think you’re probably right, though, that it has something to do with the way energy gets transferred from light to matter.
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Wow. Great post. I had no idea about any of that. What a cool and simple experiment.
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Yeah, when I stumbled upon this little story, it kind of blew my mind. It’s crazy how Herschel made such a big discovery with such a simple experiment.
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Yeah… and he’s one of these people who really made an amazing bunch of contributions to science, but everything’s forgotten because of the one biggie.
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