In 1801, after hearing about William Herschel's discovery of "heat rays" (infrared radiation), physicist Johann Wilhelm Ritter (1776-1810) looked for an opposite (cooling) radiation at the other end of the visible spectrum.
Ritter did not find exactly what he expected to find, but after a series of attempts he noticed that silver chloride was transformed faster from white to black when it was placed at the dark region of the sun's spectrum, close to its violet end.
The "chemical rays" found by him were afterwards called ultraviolet radiation.
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