Why the Sky Is Blue: The Science of Rayleigh Scattering

When you look up on a clear day, the sky appears blue. That everyday fact comes from a beautiful piece of physics called Rayleigh scattering.

Sunlight is a mix of all visible wavelengths—from violet to red. As this light travels through Earth's atmosphere, it hits countless tiny molecules of nitrogen and oxygen. James Clerk Maxwell's equations and later work by Lord Rayleigh showed that the amount of scattering depends strongly on wavelength: shorter wavelengths (blue and violet) are scattered much more than longer ones (red and orange).

The scattered blue light reaches our eyes from all over the sky, so the sky looks blue. At sunrise and sunset, sunlight passes through a much thicker layer of atmosphere. Most of the blue is scattered away before the light reaches you, so you see the remaining red and orange—giving us those dramatic sunsets.

This same idea helps explain why the ocean can look blue: water absorbs red light more than blue, so the blue is what's left to bounce back to your eye.