The existence of ultraviolet irradiation in the solar spectrum was revealed in 1801, when a Jena physician, Johann Ritter (Ritter 1801), investigated the effectiveness of different colors of light in blackening crystals of silver chloride; he showed that radiation beyond the violet was most effective. The ultraviolet portion of the electromagnetic spectrum is divided between the radiation absorbed by the air, and therefore named “vacuum” (or “far”) ultraviolet (10–200 nm), and the air-penetrating radiation of wavelengths of 200–380 nm. The latter range is subsegmented below and above ca. 300 nm. This value demarcates the upper limit for the solar wavelengths absorbed and filtered by the stratospheric ozone layer, meaning that only longer wavelengths reach the earth. It also differentiates between ultraviolet radiation of wavelengths shorter than 300 nm, which is absorbed by the fundamental components of the living cell, such as proteins and nucleic acids, and the radiation above this value which is not and is therefore less noxious.
KeywordsCholesterol Mercury Ozone Lipase Photosynthesis
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