Why the sky is blue and the sunset red: Rayleigh scattering and the length of the air
30seconds
The sky is blue and the sunset is red, yet the sunlight is the same. Air scatters short blue waves about four times more strongly than long red ones, a fourth-power rule worked out by Lord Rayleigh.
Drag the Sun up and down in a simple three-colour model and watch which colours survive the trip through the air. Slide the air thickness to zero and the sky goes black, as it does for astronauts.
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