Ultraviolet (UV) radiation is a form of light that exists just beyond the visible colors our eyes can perceive. The term “ultraviolet” translates literally to “beyond violet,” indicating its position at the short-wavelength, high-energy end of the visible spectrum. The discovery of this invisible energy in the early 19th century marked a significant expansion in the understanding of light. It proved that sunlight consisted of more than just the colors of the rainbow.
Setting the Stage: The Known Spectrum
The path to discovering ultraviolet radiation began with a related breakthrough at the opposite end of the visible spectrum. In 1800, the British astronomer Sir William Herschel conducted an experiment investigating the heating power of the different colors produced when sunlight passed through a prism. He used a prism to separate sunlight into the familiar band of colors, from red to violet, and then placed thermometers in each color to measure its temperature.
Herschel’s measurements showed a progressive increase in temperature as he moved the thermometer from the violet end toward the red end of the spectrum. Intrigued by this trend, Herschel decided to place a thermometer in the area just beyond the visible red light, where no color was apparent. To his astonishment, the temperature there was even higher than in the red light itself. This proved the existence of an invisible form of radiation, which he termed “calorific rays” and is known today as infrared radiation. This finding demonstrated that unseen energy existed outside the boundaries of the visible spectrum and inspired scientists to investigate whether a similar invisible counterpart might exist at the other extreme, beyond the violet light.
The Experiment of Johann Wilhelm Ritter
Motivated by Herschel’s discovery, German physicist Johann Wilhelm Ritter searched for a complementary invisible radiation beyond the violet end of the spectrum in 1801. Ritter designed his experiment to test the chemical activity of light, rather than its heat. He reasoned that if invisible heat rays existed on one side, invisible chemical rays might exist on the other. He directed a beam of sunlight through a glass prism, separating the white light into its constituent colors.
For his detector, Ritter utilized paper that had been treated with silver chloride, a chemical compound known to undergo a reaction when exposed to light. Silver chloride turns dark when illuminated, and scientists were aware that blue light caused a stronger reaction than red light. Ritter placed strips of this light-sensitive paper along the entire visible spectrum to measure and compare the rate of darkening in each colored region. This setup allowed him to analyze the chemical effects of light across the rainbow.
Observation of Invisible Chemical Activity
Ritter’s initial observations confirmed that the silver chloride paper darkened progressively faster moving from the red end toward the violet end of the spectrum. This result validated the understanding that the shorter-wavelength light carried more chemical energy than longer-wavelength light. The truly remarkable finding came when Ritter extended the silver chloride paper into the dark region immediately adjacent to the visible violet light.
In the space where no color was visible to the human eye, the silver chloride strips darkened even more intensely and rapidly than they had under the violet light. This observation provided undeniable proof that an invisible form of energy was radiating from the sun with a strong capacity to induce chemical change. Ritter initially referred to this new radiation as “de-oxidizing rays” or “chemical rays.” These chemical rays were later formalized as ultraviolet radiation.