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William Herschel | Philosophical Transactions of the Royal Society of London | (1800)
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Key Takeaways

Plain English Takeaway

This paper shows that sunlight contains invisible rays that can heat things up, even though we can't see them with our eyes.

Study Aim

The main goal of this paper is to investigate whether the heat from sunlight includes rays that are invisible to the human eye, and to determine if these invisible rays can be bent (refracted) by a prism in the same way as visible light. The author wants to find out if the range of these heat-carrying rays extends beyond the colors we can see. Simply put: The study wants to see if sunlight has invisible parts that can be bent like visible colors and if these parts carry heat.

Study Design

The author describes a series of experiments using a prism to split sunlight into its different colors on a piece of white paper. He marks lines on the paper to measure where the different colors fall. He then uses thermometers to measure the temperature at different spots, including areas beyond the visible red light. By comparing the temperatures, he checks if there is heat in places where no visible light appears. Simply put: The researcher shines sunlight through a prism and uses thermometers to see if there is heat where we can't see any color.

Findings

The experiments reveal that the highest temperature is found just beyond the red end of the visible spectrum, where no light is seen. This shows that there are invisible rays in sunlight that carry heat, which the author calls 'calorific rays' (heat rays). These rays can be bent by a prism, just like visible light. The findings suggest that sunlight contains more than just the colors we see; it also has invisible rays that can warm objects. The author recommends further study of these invisible rays to better understand their properties and effects. Simply put: The study finds that sunlight has invisible heat rays that can be bent by a prism and make things warm, even though we can't see them.

Abstract

In that section of my former paper which treats of radiant heat, it was hinted, though from imperfect experiments, that the range of its refrangibility is probably more extensive than that of the prismatic colours; but, having lately had some favourable sunshine, and obtained a sufficient confirmation of the same, it will be proper to add the following experiments to those which have been given. I provided a small stand, with four short legs, and covered it with white paper. On this I drew five lines, parallel to one end of the stand, at half an inch distance from each other, but so that the first of the lines might only be ¼ of an inch from the edge. These lines I intersected at right angles with three others; the 2d and 3d whereof were, respectively, at 2½ and at 4 inches from the first.

Referenced In

StarTalk

Season 17, Episode 28: William Herschel, The Man Who Saw Beyond the Spectrum

Hey StarTalkians! Season 17, episode 28 was a “Things You Thought You Knew” edition, where Neil illuminated Chuck and the rest of us on all things light. Towards the end, they briefly touched on the story of William Herschel:

Things You Thought You Knew – Is Everything Light? - StarTalk Radio

(from 39:50)

Neil gave a good overview of the experiment, but the whole story is a fantastic illustration of how far an insightful mind can take you, even with no formal scientific training. It’s been covered in-depth on StarTalk before, but in case you missed it, here’s the story.

Herschel’s Unique Path to Science

William Herschel didn’t start out as a scientist. Born in Hanover to a musical family, he was working as a music teacher in England in the late 1750s when he began to study astronomy, optics and mathematics in his spare time.

He started making his own telescopes, even discovering Uranus in 1781. But as he approached the problem of observing the sun, he asked an interesting question: do the different colours of light have different temperatures?

Taking the Spectrum’s Temperature

This led him to one of his most famous experiments. He used a prism to separate out the colours of sunlight, and shone each through a slit in a piece of cardboard, onto a thermometer beneath. As Neil describes, there was a “control” thermometer right beside it, out of direct sunlight.

The result justified his interest: the thermometer in red light increased by 6.88 °F, while for green light it only rose 3.25 °F and for violet, just 2 °F.

But Herschel didn’t stop there. He followed his curiosity: since he couldn’t identify a peak, what if it comes beyond the visible spectrum?

Seeing Beyond the Spectrum

He set up another experiment, but this time, the measurements started at the edge of the visible spectrum, as red becomes what we now call infrared (“below red”). He continued the line, tracing the rising temperature to its peak in the region “not fit for vision.”

Most remarkably of all, he was able to piece the results together, arguing (correctly) that infrared and light are ultimately the same thing, outside of our limited perception:  

“To conclude, if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be inquired, whether light be essentially different from radiant heat?”

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