Ultraviolet radiation is invisible electromagnetic radiation between visible violet light and X rays. It ranges in wavelength from about 400 to 4 nanometers and in frequency from about 10¹⁵ to 10¹⁷ hertz. It is a component (less than 5%) of the sun’s radiation and is also produced artificially in arc lamps, e.g., in the mercury arc lamp. Vitamin D is produced by the action of ultraviolet radiation on human skin and treatment or prevention of rickets often includes exposure of the body to natural or artificial ultraviolet light. The radiation also kills germs and is widely used to sterilise rooms, exposed body tissues, blood plasma and vaccines.
The German scientist Johann Wilhelm Ritter discovered ultraviolet radiation in 1801 after learning of William Herschel’s experiments that led to his discovery of infrared radiation. Herschel observed an increase in temperature beyond the red end of the spectrum of sunlight but no change in temperature beyond its violet end. Despite this, Ritter suspected the presence of invisible radiation beyond the violet end of the spectrum and so decided to test his theory with horn silver (silver chloride) as it was known to darken upon exposure to sunlight (especially in the violet end of the spectrum). In his 1801 publication, he writes the following about the experiment he conducted, “I cover a six-to-eight-inch long strip of strong white paper with freshly prepared horn silver and I let the clear solar spectrum of the prism fall on its centre in the darkened room at a distance of fifty to sixty inches. The horn silver starts blackening at a considerable distance from the outermost violet, and then some time later follows the part within the violet itself, and finally the weakest inner hue of the greenish blue begins to blacken.” He further states, “But on the red side it has become whiter, and again in such a way that the greatest effect occurs half an inch outside the outermost red, and furthermore decreases according to the same law as in the first case on the other side.”
Here is a portrait of Johann Wilhelm Ritter: