Physics
Why the Sky is Blue: Rayleigh Scattering
Quick fact
The sky is blue because air molecules scatter sunlight about 16 times more effectively at the blue end of the spectrum than at the red end.
Why this is interesting
You step outside on a sunny day and look up—why is the sky blue, not black or white, and why does it turn red at sunset?
Read the full explanation
Understanding Why the Sky is Blue: Rayleigh Scattering
Think of sunlight as a mixture of all colors. As it travels through our atmosphere, it collides with tiny gas molecules (mostly nitrogen and oxygen). These molecules act like small antennas that absorb and re-emit light in different directions. But they don't scatter all colors equally. They are much better at scattering shorter wavelengths—blue and violet—than longer red wavelengths. So while all colors are being scattered, blue light is sent in every direction much more intensely, painting the sky blue from every angle you look.
A deeper explanation
The mechanism is Rayleigh scattering, named after Lord Rayleigh. It occurs when the scattering particles are much smaller than the wavelength of light. The intensity of scattered light is inversely proportional to the fourth power of the wavelength (I ∝ 1/λ⁴). Since blue light (λ ≈ 450 nm) has a shorter wavelength than red light (λ ≈ 650 nm), it scatters about (650/450)⁴ ≈ 4.4 times more. However, the sky is not violet because the Sun emits less violet light and our eyes are less sensitive to it. This principle also explains why the Sun appears reddish at sunrise and sunset: sunlight travels through more atmosphere, scattering away the blue light and leaving the reds to reach our eyes.