William Herschel and the discovery of infra-red radiation
Physics Narrative for 14-16
In 1800, William Herschel published a series of papers describing experiments which led him to identify infra-red radiation, a form of radiation beyond the red end of the spectrum of visible light.
Background to the experiment
When visible light falls on a surface, some of it may be absorbed; the surface is warmed. A thermometer left lying in sunlight shows an increasing temperature. Today, we would say that light transfers energy from the Sun to the Earth, and when it is absorbed, energy is stored thermally in the absorbing material.
We also receive infra-red radiation and ultraviolet radiation from the Sun. However, the nature of these was unknown in 1800.
William Herschel and his sister Caroline Herschel were astronomers working in Bath. William had been using telescopes to observe the Sun. Of course, this is hazardous, so he used dark glass filters to reduce the intensity of the Sun’s rays. He noticed that even with the filters he could sense a warming effect. He also noticed that some filters seemed to allow through more of the light, while others transmitted more radiation that warmed things up. He wrote that, when observing the Sun, he used:
...various combinations of differently-coloured darkening glasses. What appeared remarkable was that when I used some of them, I felt a sensation of heat, though I had but little light; while others gave me much light, with scarce any sensation of heat.
Herschel set about comparing how well these different glass filters transmitted what he calle 'heat', or infra-red radiation. He needed to know whether his observations were being affected by the filters he was using.
Scientific observations are made using instruments. The data collected are only as good as the instruments; scientists need to get to know their instruments, so that they understand their limitations.
Question: What happens when white light is passed through coloured filters?
Answer: Light of certain colours is observed. Some colours (bands of frequencies of the visible spectrum) are transmitted, others are absorbed.
Before experimenting with his filters, Herschel needed to know about the heating effect of different colours of light. Did different regions of the spectrum have equal heating effects?
He set up an experiment in which sunlight was passed through a slit and then through a prism (from a chandelier), forming a spectrum on his table. He arranged three thermometers in such a way that the central one could be placed at different points in the spectrum. The other two were positioned on either side, to act as controls.
Question: What does ‘control’ mean here?
Answer: Herschel needed to show that any effect he observed was caused by the light falling on the central thermometer. The control thermometers eliminated any general heating of the apparatus which might have been occurring.
Herschel made repeat measurements with the thermometer bulb in the violet, green and red regions of the spectrum. In each, he observed a temperature rise, which he recorded after 8 minutes.
Average rise in red: 6.9°F, in green: 3.2°F , and in violet: 2°F.
He concluded that the red rays of sunlight have a greater heating effect or that there is more of it.
As the Sun moved across the sky, the spectrum moved across the table. When the central thermometer was just beyond the red end of the spectrum, Herschel noticed that the temperature was even higher than before. What was going on? He concluded that there were invisible rays, coming from the Sun, and refracted by the prism beyond the red end of the spectrum.
Experiment: Carry out a simulation of Herschel’s experiment. See the experiment
Question: In the light of Herschel’s discovery, what experiments would you suggest carrying out next?
Answer: Suggestions might include: seeing how far beyond the red the effect extended; testing beyond the violet, testing light from sources other than the Sun.
What Herschel did next
Herschel found that the heating effect of these invisible rays was greatest at a point beyond the red end of the visible spectrum, and gradually diminished to zero beyond this point. He also looked beyond the violet end of the spectrum, but found no measurable heating effect.
However, he was able to publish three papers reporting his results, dealing with the heating effect, reflection and refraction (‘refrangibility’) of sunlight, and showing that the same effects could be observed with light from terrestrial sources.
Being a series of papers read at the Royal Society, and published in the Philosophical Transactions.
Scientific papers, as well as being published, were often first read out at a meeting of a scientific body such as the Royal Society (in London). Philosophical Transactions is still published, and is the second oldest scientific publication in the world. Today, many scientific discoveries are first reported at conferences before being formally published.
Concluding his second paper, Herschel suggested that 'heat' and light were all part of the same spectrum, parts of which we see with our eyes, while other parts we feel as heat on the skin.
Question: What do we now call this (much extended) spectrum?
Answer: The electromagnetic spectrum.
Herschel might have continued to think of light and 'heat' as separate forms of radiation. However, he chose to regard them as two different forms of the same phenomenon. Hence, he united two apparently different phenomena, light and 'heat'. Producing explanations of physical phenomena that are as simple as possible is one of the most general aims of science. In Herschel’s words:
… we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one.
Herschel also suggested that the different colours of light might have different chemical effects, i.e. that they might have different effects in chemical reactions. He was right; this is made use of in photographs.
Once scientists have a new theory to work with, they use it to predict new effects; this suggests experiments which will help to test the theory.
Where this led
Physicists’ understanding of the electromagnetic spectrum has extended gradually over the two centuries since Herschel’s work. Applications are manifold.
Herschel was an astronomer; his work led directly to the idea that more could be learnt about the universe by detecting wavelengths other than visible light. Today, we have infra-red astronomy, X-ray astronomy and so on.
Question: Herschel used thermometers to detect and measure the heating effect of light. What is the difference between ‘detecting’ and ‘measuring’? What other instruments are used to detect electromagnetic radiations?
Answer: Geiger counters for gamma, thermochromic paint for infra-red, photographic film for UV to X-rays, aerials for radio waves, etc.
New or improved instruments can allow scientists to learn about previously undetectable or unmeasurable phenomena. For example, photomultipliers in space telescopes can measure X-rays from distant, sources across the universe, otherwise invisible.
We are grateful to David Sang, author of this Case Study.