Why the Sky Is Blue

June 1, 2026

Sunlight scatters off air molecules, and the intensity of that scattering depends strongly on wavelength. Rayleigh scattering tells us the scattered intensity goes as

Since blue light ($\lambda \approx 450\,\text{nm}$) has a much shorter wavelength than red light ($\lambda \approx 700\,\text{nm}$), it scatters roughly

times more strongly. That scattered blue light reaches your eyes from every direction in the sky — which is why the sky looks blue.

At sunset

When the sun is low, light travels through much more atmosphere. Most of the blue is scattered away before it reaches you, leaving the direct light enriched in red and orange.