The Electromagnetic Spectrum — The World, with the Evidence

A glass prism mounted on a copper tube. — Glass prism mounted at the end of a brass tube, used by William Herschel for his experiments on the heat of sunlight split into its colours (Science Museum, London). Science Museum, London, CC BY-SA 2.0, source

One day in 1800, an astronomer placed a thermometer where there was nothing to see. The mercury began to rise.¹

It is one of the simplest experiments in the history of science. The story usually told about it is a little too tidy.

How it began

William Herschel was observing sunspots through a telescope. Looking at the Sun requires its light to be filtered very heavily, or the instrument will be destroyed — and the observer’s eye with it.²

While trying out various combinations of coloured glasses, he noticed something strange, and he wrote it down himself: with some of them, he felt a sensation of heat though he had but little light; with others, much light and scarcely any sensation of heat.³

What he made of this is a model of reasoning. Since the image of the Sun was also coloured differently depending on the glasses, it occurred to him that the power of heating might be very unequally distributed among the colours.⁴

Then he went one step further, and it is this step that counts: having judged it right to entertain a doubt on this point, it appeared to him equally proper to doubt in the same way with regard to light. If some colours heat better, others perhaps illuminate better.⁵

His conclusion: at all events, it would be proper to recur to experiments for a decision.⁶

The experiment

Herschel therefore split sunlight with a prism, then measured the temperature of each colour by placing mercury thermometers in it. He found that the heating power was indeed very unequally distributed, and that red was by far the strongest.⁷

He then carried out the second experiment, the one he had announced: he looked for where the illumination was greatest, by observing small objects under a microscope in each colour. The answer: in the brightest yellow, or palest green — not in the same place as the heat.⁸

The two maxima do not coincide. Then, going further, he placed a thermometer beyond the red, where the eye no longer perceives anything.⁹

Absolute safety rule: never look at the Sun, whether with the naked eye, through an instrument or through a filter. The prism projects the spectrum onto a neutral surface — a white sheet — and it is this surface that is observed. This is exactly how Herschel placed his thermometers.

The mercury rises. Even higher than in the red zone of the visible spectrum.

Herschel had just measured an invisible radiation: what we now call infrared.

The simplified version (and what it glosses over)

This experiment is often told as follows: Herschel supposedly placed this thermometer outside the spectrum by pure chance or simply as a control, to check that nothing was happening in the dark. On this account, the discovery was sheer coincidence.

This romanticised version is appealing, but it glosses over the real driving force of the scientific approach.

Herschel was not searching blindly. He had a precise reason to suspect that the heat did not follow the distribution of the colours: his coloured glasses had alerted him to it, and he wrote it down in black and white before he began.

This in no way diminishes the significance of his discovery. It makes it understandable — and shows that decisive experiments arise from an anomaly that an attentive mind refuses to ignore.

The simplified version also passes over a second crucial point. To say that “he had just discovered an invisible light” is our modern way of framing the event. Herschel himself first believed that heat and light were one and the same thing. The same year, in the second part of a long paper, he concluded the opposite: they are two distinct entities. His observation was rigorously accurate; the interpretation he drew from it differed profoundly from ours. He concluded — wrongly — that this radiation was probably not light. That is the most common situation in the history of science.¹⁰

What this discovery actually produced

This experiment is often taught as proof that our senses do not set the bounds of the world: there are physical realities that we do not perceive directly. That is true, and it is the meaning we give it today.

Yet in its own day, it served to support quite a different thesis.

At the time, the scientific community regarded heat as an invisible fluid: caloric (see our page 4.9 on heat). Light was then believed to be made of corpuscles. Discovering that heat travels through space in the same way therefore suggested that it, too, was made of particles: particles of caloric crossing the void.

Herschel’s experiment thus became an argument in favour of the caloric theory.¹¹

In the 1820s, when physicists finally adopted the wave nature of light (thanks to work carried out over the two preceding decades), this argument collapsed. But the caloric theory itself survived for another twenty years or so.¹²

Beyond red, and much further still

Since Herschel’s discovery, physicists have established that visible light is only a tiny fraction of a vast family of radiation. All share the same nature — electromagnetic waves travelling at the speed of light (about 300,000 km/s in a vacuum) — and differ only in their wavelength (or their frequency).¹³

From the longest radiation to the shortest:

Radio waves — Used for telecommunications, radio and mobile telephony.
Microwaves — Put to work in household ovens, radar and Wi-Fi.
Infrared — The thermal radiation discovered by Herschel, used notably in remote controls and thermal cameras.
Visible light — The only band perceived by the human eye, from red to violet.
Ultraviolet (UV) — More energetic radiation, responsible for tanning and sunburn.
X-rays — Able to pass through soft tissue, widely used in medical imaging.
Gamma rays — Extremely energetic radiation emitted by the decay of atomic nuclei and by violent cosmic events.

In everyday life we are bathed in several of these kinds of radiation while perceiving almost none of them. The visible band is only a minute slice of the whole: the infrared range alone is hundreds of times wider.

The lesson is the same in acoustics: we hear only a narrow band of vibrations (see page 4.11). Same principle, two different senses: our organs of perception do not define the limits of reality.

What this explains in the rest of the site

This page gives a direct answer to questions that arise when reading our chapters on astronomy:

Why are the images from astronomical observatories described as translations (1.13)?
Why is the colour given to the first image of a black hole the result of a deliberate choice (1.10)?
Why do photographs of Neptune have to be enhanced by image processing (1.8)?

Because most of the signals picked up by modern telescopes are completely invisible. They are radio waves, infrared, ultraviolet or X-rays.

To make these data accessible to human vision, scientists assign visible colours to this invisible radiation. This is not falsification, but a visual translation — and like any translation, it must be explicitly documented.¹⁴

Note 1

Read April 24, 1800. […] went on to the next experiment, by withdrawing the stand, till the whole ball of No. 1 was completely out of the sun's visible rays

Herschel, William, “Experiments on the Refrangibility of the Invisible Rays of the Sun”, Philosophical Transactions of the Royal Society, vol. 90, 1800, p. 284-292

See the full note ↓

Note 2

During years of solar observations, he had used pieces of colored glass to reduce the amount of light and heat coming through the telescope.

American Physical Society, “Herschel Proposes the Existence of Invisible Light”

See the full note ↓

Note 3

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.