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Not all rays have the same wavelength
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The solar rays reach the Earth's surface after being attenuated by the atmosphere. Made up of elementary particles called photons, they are characterized by a wavelength inversely proportional to their energy - the shorter the wavelength, the greater the energy.
The skin reflects some of the rays and its mixed structure enables it to deflect others towards the upper levels of the epidermis. But care should be taken, since the remainder are absorbed.
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UVC, UVB and UVA rays, of progressively longer wavelength, are the three types of rays making up the ultraviolet
spectrum (100 to 400 nm).
UVC rays (100 to 280 nm) are stopped by the ozone layer and do not reach the Earth's surface. UVB rays (280 to 320 nm) and UVA rays (320 to 400 mm) account for 10 % of the solar rays reaching the Earth's surface. But their levels are not constant - they depend on latitude, season, atmospheric conditions and above all on the hour of day.
70 % of the UVBs are reflected by the horny layer. 30 % of them penetrate the epidermis, where they are partially absorbed by the keratinocytes and melanin, only 10 % of them reaching the upper dermis. Because of their short wavelength, the UVB rays have a high energy level and are therefore biologically very active, causing sunburn. UVA rays are lower in energy than UVBs, but they are up to 100 times more abondant, with 80 % of them reaching as far as the dermis, and 20 % of these penetrating very deeply. In contrast with UVB rays, glass is not an effective barrier against them and their effects are not immediately visible, so that they are much more dangerous.
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Visible light spectral decomposition
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The greater part of the visible light
(400 to 800 nm) penetrates the atmosphere, and if its path is not stopped by dust or smoke, it accounts for 40 % of solar rays at ground level, of which 20 % penetrate the three skin compartments. This is shown by observing the hand's transparency in front of a lamp, when the fingers appear red, coloured by the blood flowing through the blood vessels.
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50 % of solar rays reaching the Earth's surface are infrared (IR) (800 to 3200 nm). As they reach the hypodermis, they release large quantities of energy which is transformed into perceptible warmth.
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