Why the Sky Is Blue and Sunsets Are Red
The sky is not a painted surface. It is air, lit by the sun, and the colour you see is the light that was scattered toward you instead of passing straight through.
Rayleigh scattering
Air molecules are far smaller than the wavelength of visible light. When light meets particles that small, it scatters, and the amount of scattering rises sharply as wavelength falls. Blue light, at the short end of the visible spectrum, scatters far more than red. So blue light is redirected across the whole sky, and that redirected light is what reaches your eye from every direction.
Why sunsets turn red
At sunset, light travels a much longer path through the atmosphere. Most of the blue has already been scattered out of the beam, leaving the longer wavelengths — orange and red — to come through directly. Dust, smoke and humidity strengthen the effect by scattering and absorbing more of the short wavelengths.
Why the sky is not violet
Violet scatters even more than blue, so why not a violet sky? Two reasons: the sun emits less violet than blue, and human eyes are less sensitive to violet. The mixture our visual system reports is blue.
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Why is the sky blue rather than violet?
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