Ultraviolet (UV) light is a form of electromagnetic radiation that exists just outside the range of light visible to the human eye. This invisible radiation is positioned at the higher-energy, shorter-wavelength end of the spectrum, immediately following the color violet. The discovery of UV light was the second half of a scientific revolution that demonstrated light extended far beyond the familiar colors of the rainbow. The search for this unseen energy began with a similar investigation at the opposite end of the solar spectrum.
Mapping the Solar Spectrum
The groundwork for discovering invisible light was laid in 1800 by the British astronomer William Herschel. He was investigating how much heat was contained within the different colors of sunlight. Herschel directed sunlight through a glass prism to separate it into the visible spectrum, creating the familiar band of colors from red to violet. He then placed sensitive thermometers across this spectrum to measure the temperature of each color.
Herschel observed that the temperature steadily increased as he moved the thermometer from the violet end toward the red end of the spectrum. He moved the thermometer just past the red light, into a region that appeared completely dark. In this invisible area, the thermometer recorded the highest temperature of all, proving that a form of unseen radiation was carrying heat. Herschel initially named this energy “calorific rays,” which are known today as infrared radiation.
Identifying the Chemically Active Rays
Inspired by Herschel’s demonstration that invisible radiation existed beyond the red end of the spectrum, the German physicist Johann Wilhelm Ritter began his own experiments in 1801. Ritter hypothesized that if “heat rays” existed on one side of the visible spectrum, a similar form of energy might exist beyond the violet light. His goal was to find a type of radiation that was chemically active rather than thermally active.
Ritter used silver chloride-soaked paper as his detector, a substance known to darken when exposed to light. By placing strips of this paper across the spectrum created by a prism, he observed that the paper darkened most rapidly in the violet region. Crucially, when he placed the paper into the dark area just beyond the visible violet light, the chemical reaction intensified. The silver chloride turned black even faster than it did under the violet light itself.
This intense darkening proved the existence of a second form of invisible radiation, which had a greater capacity to drive chemical change than visible light. This radiation, which we now call ultraviolet light, was detected by its ability to cause a photochemical reaction, unlike Herschel’s heat-based discovery. Ritter’s experiment definitively established that the solar spectrum was wider than the human eye could perceive, extending into both lower-energy and higher-energy regions.
Early Understanding and Terminology
Ritter’s discovery immediately sparked a discussion about how to classify the two newly found invisible energies. Because the radiation beyond the violet end was effective at causing chemical change, Ritter and his contemporaries initially referred to them as “chemical rays.” This term served to distinguish them clearly from the “calorific rays”—Herschel’s heat-carrying radiation found beyond the red end.
The distinction between the two invisible types of light was based on their primary effect: chemical rays caused reactions, and calorific rays caused a temperature rise. As the understanding of light evolved, it became clear that both types of radiation were part of a single, continuous electromagnetic spectrum.
The modern terminology eventually took hold, based on the location of the radiation relative to the visible spectrum. Herschel’s calorific rays became known as infrared, meaning “below red.” Ritter’s chemical rays were renamed ultraviolet, meaning “beyond violet.” This nomenclature provided a universal way to categorize the expanded spectrum of light, which contained energy far beyond the limits of human vision.