Why is the sky blue?

Sunlight is white — a mix of all colours. So why does the daytime sky look blue, and why do sunsets turn red?

Air molecules are far smaller than a light wave, and such tiny obstacles scatter short, choppy waves much more than long, lazy ones. Blue light has a short wavelength, so it gets knocked sideways all over the sky; red mostly sails straight through.

Look anywhere except at the sun and what reaches your eye is that sideways-scattered light — mostly blue. At sunset, sunlight travels a much longer slanted path through air, the blue gets scattered away before reaching you, and what's left of the direct beam is red and orange.

This is Rayleigh scattering: for particles much smaller than the wavelength, scattered intensity is proportional to 1/λ⁴. Blue light (~450 nm) is scattered roughly (700/450)⁴ ≈ 5.6 times more strongly than red (~700 nm).

The sky isn't violet, even though violet has a still shorter wavelength, because sunlight contains less violet and our eyes are much less sensitive to it — the scattered mixture we perceive is blue.

Clouds are white for a different reason: their droplets are much larger than a wavelength, and large-particle (Mie) scattering treats all colours nearly equally.

Compare noon with sunset. At noon, the sun's light crosses a relatively short column of atmosphere, so only part of the blue is scattered out — the sun looks white-yellow against a blue sky. At sunset the path length through air is many times longer; by the time the beam reaches you, most short wavelengths have been scattered away, leaving the disc and nearby sky red-orange.

• Saying the sky is blue 'because it reflects the ocean' — the effect works over deserts and for astronauts' photos of the atmosphere alike; it's scattering by air itself.
• Thinking blue light is absorbed rather than scattered: the blue photons aren't destroyed, they're redirected — that's precisely why they reach you from every part of the sky.

• NCERT Class 10 Science, Ch. 10 — The Human Eye and the Colourful World
• NASA Space Place — Why is the sky blue?