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Why is the sky blue?

According to Raleigh Law, the intensity of scattered light is inversely proportional to the fourth power of wavelength.
As the wavelength of blue color is smallest and that of red is longest, much of the shorter wavelength blue light is radiated in different directions. Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun's light is predominantly in the blue end of the spectrum.

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