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?”