The sky is blue because air scatters short wavelengths of light the most

Sunlight looks white, but air molecules scatter its blue end far more than its red end — a wavelength-dependent effect worked out by Lord Rayleigh in the 1870s. Look away from the sun on a clear day and you're seeing that scattered blue light arriving from every direction at once.

— Lord Rayleigh (John William Strutt), On the light from the sky, its polarization and colour — Philosophical Magazine, 1871