<prompt> Which of the following ultra-violet radiation cause most skin disorder ?UV-AUV-BUV-CNone  <response>: Ans. is b i.e., UV-B Ultraviolet radiation UVR UVR is electromagnetic radiation with wavelength 200-400 nm shoer than visible light, but longer than x rays. There are three segments of UVR. 1. UV-CSho wave UV-radiation 200-290 nm It is most dangerous as it has serious effects on the skin. But, It is not medically impoant as it is absorbed by ozone layer and hence does not reach the surface of eah in measurable amounts. 2. UV-Bmedium wave UV-radiation 290-320 nm Medically most impoant UVR as it causes most of the dermatoses, e.g., sunburn, tanning, photoaging. It is absorbed in epidermis. It is used in phototherapy as narrow band UV-B NBUVB which has wave length of 311 nm. 3. UV-ALong wave UV-radiation 320-400 nm It is absorbed in dermis. It is fuher divided into : ? UVA-2 --- 320-340 nm UVA-1 --- 340-400 NM Woods lamp uses UVA-1365 NM. UVA is also used in photochemotherapy PUVA. 