Atmospheric scattering
Drag the sun across the sky and watch the atmosphere change color in real time. Rayleigh scattering — the interaction of sunlight with air molecules — scatters short wavelengths (blue) far more than long ones (red). At noon the sky is blue; at sunset the light path is long enough that only red survives.
Intensity ∝ 1/λ&sup4; — Rayleigh scattering law
Why is the sky blue?
Sunlight entering the atmosphere collides with nitrogen and oxygen molecules. These molecules are much smaller than the wavelength of visible light, so they scatter it according to Rayleigh’s law: the intensity of scattering is proportional to 1/λ&sup4;, where λ is the wavelength. Blue light (around 450 nm) scatters roughly 5.5 times more intensely than red light (650 nm).
Why are sunsets red?
At sunset, sunlight travels through much more atmosphere to reach your eyes. The longer path means almost all the blue light has been scattered away before arrival. What remains is predominantly red and orange — the wavelengths that survive the long journey through the scattering medium.
The atmosphere density slider
Increasing atmosphere density simulates a thicker or denser atmosphere — like that of early Earth or another planet. Higher density means more scattering at every angle, intensifying colors at noon and shifting sunset colors further toward deep red.