Why is the daytime sky blue but the Sun turns red at sunset?
Air molecules scatter shorter (blue) wavelengths far more than longer (red) ones — Rayleigh scattering, which goes as 1/λ⁴.
Air molecules scatter shorter (blue) wavelengths far more than longer (red) ones — Rayleigh scattering, which goes as 1/λ⁴. By day, scattered blue light reaches your eyes from all directions, so the sky looks blue. At sunset the light travels a long slanted path through the atmosphere; most blue is scattered away before it reaches you, leaving the reddened Sun.
Key point
Rayleigh scattering (∝ 1/λ⁴) scatters blue most; long sunset path leaves red.
Common mistake
thinking the sky is blue because it reflects the sea.
Memory tip
blue scatters most (blue sky); at sunset it's scattered out (red Sun).
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