Rayleigh scattering plus how the eye is tuned; the usual answer is only half of it
Violet light scatters more than blue in the atmosphere, but the sky looks blue because the Sun emits fewer violet photons and because the human eye is far less sensitive to violet. Neither fact alone is enough; the perceived color is the product of both.
Rayleigh scattering by air molecules is strongly frequency-dependent: the scattered intensity is proportional to the inverse fourth power of wavelength. Compare blue at 450 nm and violet at 400 nm. The ratio is (450/400)^4, about 1.6. So a violet photon is roughly 1.6 times more likely to be scattered than a blue photon. But “more likely per photon” is not the whole story.
Because of the Sun’s surface temperature and atmospheric absorption, ground-level daylight contains more blue photons than violet photons. Take a simple case: suppose a patch of sky receives 100 blue photons and 80 violet photons. After Rayleigh scattering, the violet contribution is 80 × 1.6 = 128 scattered photons, while the blue is 100 × 1 = 100. Scattered violet still wins in raw photon count.
But the human eye is not a photon counter. Its sensitivity falls sharply at the short-wavelength end of the visible spectrum. At 400 nm, the visual response is only a small slice of what it is at 450 nm. The 128 violet photons therefore produce a much weaker neural signal than the 100 blue photons. The brain’s verdict is blue.
These numbers are illustrative, not measured constants. The ordering is what matters: scattering favors violet, the solar spectrum favors blue, and the eye favors blue even more strongly.
Many people think air itself is a blue material, like a film of dye. It is not. A single air molecule is colorless; the color emerges from scattering, which redirects existing sunlight into your line of sight. If you look away from the direct Sun, what you see is light that has been bounced down to you. That is why the sky is brighter near the horizon than overhead: there are more molecules along that sightline and more scattering opportunities, even though the overhead path is thinner and produces a deeper blue.
Rayleigh scattering explains a clear sky when particles are much smaller than the wavelength. When clouds, haze, or pollution introduce droplets and aerosols comparable to the wavelength, Mie scattering takes over. Mie scattering barely distinguishes blue from violet, so all wavelengths scatter roughly equally, making clouds and smog look white or gray. On Mars, fine dust in the atmosphere scatters red light forward, giving a butterscotch sky; a mostly gas atmosphere is needed for the blue we see on Earth. The simple Rayleigh picture also fails at sunset if the air is very humid or smoky, which adds a grayish or pale cast to what would otherwise be a crisp red.
Cram Everyone says the sky is blue because of scattered sunlight. Sounds too simple.
Rep The simple answer is right but incomplete. Rayleigh scattering hits short wavelengths hardest, so violet and blue scatter the most.
Cram Violet is shorter than blue. Should the sky be violet then.
Rep Two reasons it is not. First, the sun gives less violet light than blue. Less to scatter.
Cram Less violet from the source. What is the second reason.
Rep Your own eye. Human cone cells detect blue far better than violet. Your brain says blue.
Cram So the atmosphere scatters violet most, but my eye barely registers it.
Rep Physics says violet should win. Biology says blue. Together they make the sky you see.
Cram What about sunsets. Why do they turn red.
Rep Same scattering, longer path through air. Blue and violet scatter away completely before reaching you. Only red and orange survive.
Cram One mechanism explains both the blue sky and the red sunset.
Rep One mechanism, two outcomes depending on path length. That is what a good model does.
Cram Full picture: violet scatters most, but the sun gives less and my eye ignores it.
Rep Now you have it. The simple answer was not false. Just half the story.