9 months ago
Last edited at 6:43AM on 6/23/2013
The sky is blue because if the competing effect of two effects: (a) the scattering of sunlight off small atmospheric particles, known as Rayleigh scattering, that favors shorter wavelengths. In fact, the scattering amplitude scales with the frequency raised to the 4th power. Therefore violet light is scatters 16 times as much as red light. If that were the end of it, the sky would be violet. And (b) This effect competes with the spectrum of sunlight, which peaks at green, lower frequency than blue. Although violet is scattered the most, there are many more photons in green than there are in violet. Blue, in between green and violet, is the winner: more photons than in violet (although not as many as green), and more scattering than green (although not as much as violet).