The Mechanism
In 1800 the astronomer William Herschel, curious about which colors of sunlight carried the most heat, split sunlight with a glass prism and laid thermometers with blackened bulbs across the resulting spectrum, using thermometers outside the light as controls. He found the temperature rose steadily from the violet end toward the red end. Then he moved a thermometer just *beyond* the red — into a region where the eye saw nothing at all — and it recorded the highest temperature of all. Herschel concluded that the Sun emits "invisible rays" that behave like light (he later showed they reflect and refract) but lie past the visible red; today we call this infrared radiation. He reported the work in a series of 1800 papers to the Royal Society, the first evidence that the spectrum extends beyond what human eyes can see — the opening of the entire electromagnetic spectrum.
Why It Matters
The surprising part is that Herschel was not looking for a new type of light. He was trying to learn which colors carry the most heat, and his answer led him past the visible spectrum entirely. A thermometer placed where no color could be seen still warmed more than the thermometers in the colored bands. That meant there was radiation from the Sun that did not reach human sight but still behaved like light, including reflection and refraction. This was the first clear evidence that the spectrum continues beyond red and helped open the modern idea of the electromagnetic spectrum.
Wait — That's Not Quite Right
A common mistake is to think 'invisible' means 'not there.' Herschel's work showed the opposite: something can be invisible to our eyes and still be detectable with instruments. Another misconception is that infrared is just the same as heat. Infrared can carry heat, but it is also a form of radiation that sits just beyond visible red, like other parts of the spectrum sit beyond violet.
Vocabulary
- William Herschel
- prism
- spectrum
- thermometers
- control
- visible light
- infrared radiation
- electromagnetic spectrum
- refraction
- reflection
- blackened bulb
Quick Quiz
5 questions · For classroom or kitchen table
The Experiment
Map the Warm Side of Color
On a sunny day, place a sheet of dark paper or cardboard near a window or on a sunny porch. Use a safe thermometer if you have one, or simply use your hand held nearby, not touching anything hot, to compare warmth in different spots of sunlight and shade. If you have a glass prism or a clear glass of water, you can make a small rainbow and then look for the brightest and warmest areas near the red end of the light. The goal is not to copy Herschel's exact setup, but to notice that light and heat do not always match what your eyes expect.
Ask an adult to help you keep the setup safe and steady. Try comparing three places: full sunlight, colored light if you can make it, and shade just beside the light. Record where the warmth feels strongest. Herschel used thermometers to measure what his eyes could not see, and your job is to think like a careful observer: where does the evidence point, and what might be happening beyond the part of the spectrum you can see?
dark paper or cardboard, safe thermometer if available, clear glass of water or glass prism if available, notebook and pencil, adult supervision
Where this came from
- William Herschel, "Experiments on the Refrangibility of the Invisible Rays of the Sun," *Philosophical Transactions of the Royal Society of London*, 90 (1800), 284–292. Overview: American Scientist, "Herschel and the Puzzle of Infrared" (https://www.americanscientist.org/article/herschel-and-the-puzzle-of-infrared); APS News, "Herschel Proposes Existence of Invisible Light" (https://www.aps.org/apsnews/2024/02/william-herschel-invisible-light).
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