Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays.

Not perceivable by human eye, ultraviolet rays were discovered in the portion beyond the visible spectrum in the area of shorter wavelengths, that is beyond the violet. They were observed for the first time by J.W. Ritter and W.H. Wollaston between 1901 and 1902, but their action on photographic emulsion was verified only 40 years later by H. Becquerel. The lower limit of wavelengths, initially placed at 200 nm, was then lowered first to 120 nm by Schumann, then to 60 by T. Lyman and to 13,6 by R.A. Millikan. Subsequently, emission lines in the ultraviolet were also observed at 2.8 and 2.1 nm, but in reality we pass seamlessly from the band of the most energetic ultraviolet rays to that of X-rays of longer wavelength. Since the two radiations can be distinguished by the mode of production, it also appears that the two wavelength bands partially overlap.

Based on wavelength, ultraviolet radiation can be classified into near (between 380 and 200 nm), far (between 200 and 10 nm), and extreme (less than 10 nm). The main sources of ultraviolet radiation are the Sun, mercury vapor lamps and generally gas discharge tubes. In addition, ultraviolet radiation can be emitted by luminescence (fluorescence and phosphorescence) by exciting certain substances with radiation of shorter wavelengths, such as X-rays. Ultraviolet radiation is strongly absorbed by the atmosphere, to an increasing extent as the wavelength decreases.

The ultraviolet spectrum is due for the most part to electronic transitions; the emission spectrum is obtained by introducing, for example, a metal in the flame of a gas burner, or in an electric arc; in the first case the spectrum is relatively simple, in the second case much more complex. The absorption spectrum, obtained by crossing a solution of the substance under examination by radiation of suitable wavelength, is very important for chemical analysis, because, for the molecules of organic compounds, there is absorption only for electrons of double or triple bonds, whose presence is thus highlighted.

The human eye is not sensitive to ultraviolet radiation, but their observation can be made with a screen of luminescent material, whose luminescence is excited by ultraviolet radiation. Ultraviolet detection is based on the fact that, up to wavelengths of 230 nm, the radiation impresses the photographic plate. For longer wavelengths you can use special emulsions, or interpose, as mentioned above, a fluorescent screen.

Scientific applications and biological effects

Scientific applications of ultraviolet include mineral analysis by fluorescence, microphotography for metallographic use, chemical analysis. Industrial applications include the localization of surface defects of metals, using fluorescent inks, air sterilization, the use of radiation in photochemistry for their catalytic action, qualitative and quantitative chemical analysis.

Medical applications exploit the stimulating action of radiation on metabolism, on the activity of the pituitary gland, adrenal, thyroid, marrow and gastric secretion. The biological effects of ultraviolet radiation, not considering the physiological photoreactions of plants (such as chlorophyll photosynthesis), can be divided into physiological and pathological. Physiological effects consist of photoreactions and photoperiodism. The pathological effects consist mainly in mutagenic action of genetic type, whose maximum corresponds to the peak of absorption of deoxyribonucleic acid, with consequent effect on chromosome structure.

There are also direct destructive actions on cells and carcinogenic action. Ultraviolet radiation can, finally, be the cause of erythema and hyperpigmentation of human skin, conjunctivitis and hyperconjunctivitis of the eye.

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