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Light that we can see is a type of electromagnetic radiation. The whole electromagnetic spectrum consists of waves of different wavelength ranging from the very long-wavelength radio waves to the short wavelength gamma radiation. In decreasing order of wavelength these are radio waves, microwaves, infrared light, visible light, ultraviolet light, X-rays and gamma rays. In this, what we call light encompasses the triad of infrared, visible and ultraviolet radiation. Ultraviolet light, or UV light, is light of wavelengths sorter than we can see as humans (10 nanometres to 400 nanometres). UV light is further subdivided based on its properties as we humans experiences it into several types. Among these are UV-A light whose wavelength spans the range 315-400 nanometres; UV-B light, spanning 280-315 nanometres; and UV-C, spanning 100-280 nanometres. Among these UV-B light is interesting because it is in the range that our bodies are sensitive to. As the light from the Sun passes through the atmosphere, the mixture of ozone, water vapour, oxygen and carbon dioxide present in the atmosphere absorbs all of UV-C and most of UV-B light. UV-A filters through.
Fortunately, or perhaps of the way we have evolved, UV-A is the least harmful of these types of radiation. UV-C is quite harmful but is entirely filtered out by the ozone layer. UV-B penetrates the skin to a short extent, causes delayed sunburn and also helps in synthesis of vitamin D. When exposed to sunlight, a compound called 7-dehydroxycholesterol (7-DHC) which is also known as provitamin D3, is converted into previtamin D3. This in turn isomerises into Vitamin D3. The previtamin D3 and Vitamin D3 absorb UVB light and produce substances useful for the body. For this reason, Vitamin D is also known as the sunshine vitamin.
Vitamin D is known to help in regulating calcium and phosphate metabolism. Thus, deficiency of vitamin D can lead to illnesses such as soft bones in children and osteoporosis in adults, among others. Vitamin D supplements and sensible exposure to sunlight can help avoid these conditions.
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