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As some of you may know, I’ve been working on a textbook on Electromagnetic Optics for a year now, and am near the end of the process. In finishing it, I wrote an introductory chapter that reviews the whole electromagnetic spectrum, from radio waves to gamma rays, and gives an overview of the properties of each band and the history of its discovery.

To write the history, I went back and referred to my book on the history of invisibility (which I’m sure you’ve heard me talk about before). When I got to the discovery of ultraviolet light, however, I noticed that I didn’t include a reference to the original paper of the discovery, which is odd, because I always try to do so. So, this led me to track down that original source (and explains why I didn’t originally include it).

So let’s talk a bit about the discovery of ultraviolet light, and share in its entirety the first announcement of the discovery!

Ultraviolet light was discovered as an almost immediate consequence of the discovery of infrared radiation by the German-born British astronomerWilliam Herschel in 1800. Herschel’s discovery in fact followed from his own astronomical observations of the sun. To look at the sun, Herschel had to use filters to heavily attenuate the bright sunlight, and he tried filters of various colors. Curiously, as he later noted,

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.

In particular, red-colored filters blocked a lot of light, but seemed to let lots of heat through. This led Herschel to perform a series of experiments to study the heating and illumination powers of the different colors of sunlight. He set up a simple and ingenious method to test the heating powers: he separated collimated sunlight into its different colors using a prism, and successively put a thermometer bulb into each color to see how much it raised the temperature. His original illustration of this experiment is shown below.

Herschel noticed that violet light, at one end of the spectrum, produced no perceptible heat at all, but red light, at the other end of the spectrum, produced the greatest amount of heat. In a flash of inspiration, Herschel then placed the thermometer bulb past the red end of the visible spectrum, where there appeared to be no light. He was amazed to see that there was even more heat where there seemed to be no light at all! Apparently there were invisible rays of some sort, beyond the red end of the spectrum. William Herschel had discovered infrared light.

Herschel’s discovery spread rapidly through the scientific community. One natural question that followed: might there be some sort of invisible rays on the other side of the visible spectrum, past violet? Herschel himself suspected this might be true, and he searched for heating rays past violet, without success. But another man had an even stronger motivation for the search, and his efforts would prove fruitful.

Johann Wilhelm Ritter was a German scientist and philosopher, and it was his philosophy that would drive him. Ritter was a proponent of the popular German movement known asNaturphilosophie that began in the late 1700s. Naturphilosophie in a sense might be viewed as a precursor to modern attempts to find a unified theory of physics, as the philosophy viewed all of nature as a unified whole, and sought to show connections between disparate phenomena. It also, however, stressed the importance of “intuition” over experimentation, an idea that has definitely fallen out of favor in physics.

Proponents of Naturphilosophie believed in a polarity of nature. Just as there are positive and negative electric charges, and positive and negative magnetic poles, Naturphilosophie followers believed that polarity was a general feature of nature. When Ritter learned of the discovery of infrared light, it was “natural” for him to imagine that as one “pole” of a physical phenomenon, with the other pole some sort of opposite effect beyond the violet.

Ritter first looked for some sort of invisible “cooling” rays that were the opposite of infrared heating rays. Failing to find these cooling rays, Ritter investigated whether other light-based phenomena might be affected beyond the violet end of the spectrum. In particular, it was well-known at the time that some chemical reactions could be driven by illumination; this would of course soon lead to the invention of the first photographs. In particular, silver chloride was known to blacken on exposure to sunlight, so Ritter placed the chemical beyond the violet — and found that it changed color even faster than it did when exposed to visible light. Ritter had discovered what he called “actinic rays,” which we now know as ultraviolet light.

But what was the first announcement of this discovery? It actually took a little doing to track it down, but here is a photo, in its entirety, of the announcement, which appeared in the 1801 edition¹ of Annalen der Physik. It had no title, and was just a short letter combined with another letter by another scientist:

For a translation, I turn to a 2009 paper by Frercks, Weber and Wiesenfeldt²,

On 22 February, I also encountered rays alongside violet in the colour spectrum of colours—outside it—by means of horn silver. They reduce even more strongly than violet light itself, and the field of these rays is very wide (cf. Annal[en der Physik], 1801, VII, 149, note). More to come soon. Ritter.

I find it quite remarkable that such a major discovery was announced in such an offhand and brief way! This also explains why I did not find and cite this paper when I was working on my invisibility book — very few people seem to have bothered including this original reference, and I thus had a hard time finding it.

Despite being such a short statement, the discovery was another revolution in our understanding of the nature of light, and physics overall. I’m glad I took the time to track down the original source!

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1. Ritter, J. W. (1801a). [Am 22sten Febr . . .]. Annalen der Physik, 7, 527.
2. J. Frercks, H. Weber, G. Wiesenfeldt (2009), “Reception and discovery: the nature of Johann Wilhelm Ritter’s invisible rays,” Studies in History and Philosophy of Science Part A, 40, 143-156,

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