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Source for Sticker Labels |
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| Release time:2013-10-07 Source:admin Reads: | |
| The sun emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet sticker label where it crosses into X-rays at 10 nm. Extremely hot stars emit proportionally more UV radiation than the Sun. For example, the star R136a1 has a thermal energy of 4.57 eV, which falls in the near-UV range (optically, such stars appear blue-white rather than violet). Sunlight in space at the top of Earth's atmosphere, at a solar constant output of about 1366 watts/m2, is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet sticker label light, for a total ultraviolet power of about 140 watts/m2 in vacuum. However, at ground level total sunlight power decreases to about 1000–1100 watts/m2, and by energy fractions, is composed of 44% visible light, 3% ultraviolet (with the Sun at its zenith), and the remainder infrared.Thus, sunlight's composition at the zenith at ground level, per square meter, is about 527 watts infrared radiation, 445 watts visible light, and 32 watts UV. Since with the Sun at zenith the Earth's air and ozone layer allows passage of a total of 32 watts/m2 (ground UV power) out of a vacuum value of about 140 watts/m2 (i.e., 23%) of Sun's UV light, this is equivalent to a minimal atmospheric blockage of 77% of the Sun's UV sticker label. However, most of the Sun's UV that is blocked by Earth's atmosphere lies in the shorter UV wavelengths. The figure rises to 97–99% of the Sun's UV radiation at the average mixture of other Sun angles encountered through the day. Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm. |
