actinic rays defined in 1939 yearactinic rays - Actinic Rays (Gr. aktis, ray);
actinic rays - Bays of sunlight which do not give out heat, but produce chemical changes, as in photography. A beam of sunlight is made up of rays which range in colour from red to violet. Beyond these red rays are invisible rays called infra-red, and beyond the violet other rays, also invisible, known as ultra-violet. The infra-red rays and those in their neighbourhood were regarded as those giving out heat and were thus called calorific rays. The rays at the other end of the spectrum, the ultra-violet, which are of much shorter wave length, do not give heat, but are much more active in producing chemical changes. They were first called actinic or chemical rays by Sir John Herschel, and before photographic emulsions were made sensitive to red and infra-red rays, the term was applied only to the blue, violet and ultra-violet. The ultra-violet rays have also been called Ritterian rays after their discoverer, Hitter. See Infra-red: Light.
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