The sky that is surrounded by the earth's atmosphere, itself is not light, the color of the sky is the effect of sunlight through the atmosphere effect on it. Because of containing gas molecules in the air, a large number of dust and ice crystals and water droplets and other particles, so, when the light from the sun through the atmosphere, will meet the particles in the air, these particles of light scattering effect. Sunlight contains all the visible light wavelength range. Often said that the sun's rays include red, orange, yellow, green, green, blue, violet seven colours, including red light wavelength, the longest shortest violet light wavelengths. Mainly air molecules in the sunny sky, it for red and orange light scattering with long wavelength is less, strong ability to penetrate, through more, but mainly through the atmosphere to the ground, and the shorter wavelengths of light green, blue, and purple, easily by gas molecules are scattered in the air, scattering is more, penetration ability is weak, directly through the relatively small. And because the purple light is absorbed through the atmosphere relatively more, less sensitive to them, and human eyes so clear sky show blue color. YanZhiXiang
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