Nib asked for
Why Not Violet
Violet scatters even more than blue, but the sun gives off less of it and our eyes respond weakly to it, so the sky we perceive comes out blue.
4 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1scatter strength
The last page ended with a rule: the shorter the wavelength, the more strongly air scatters it. Violet has the shortest wavelength of any visible color, shorter than blue. So by that rule, violet should scatter even more than blue, and the sky should look violet. It doesn't, and the reasons are about the light source and about us, not about the air.
By scattering alone, violet should win. It doesn't, so something else is shaping what we see.
a common mix-upThe sky is blue simply because blue is the most scattered color.
If scattering were the only thing that mattered, which color would the daytime sky be?
Show the answer
Violet, since it has the shortest wavelength and scatters the most.
2less supply
The first reason is the sunlight itself. The sun does not emit every color equally. At the top of the atmosphere it puts out noticeably less violet than blue. So the violet being scattered starts from a smaller supply. Strong scattering of a weak input gives a modest output.
Sunlight contains less violet than blue, so there is less violet to scatter.
3our eyes
The second reason is us. Our eyes detect color with three kinds of cone cells, and none of them responds strongly to violet. Blue light stimulates the eye far better. So even the violet that does arrive barely registers compared with the same amount of blue. What we see is not the raw light but the light as our eyes weigh it.
Our eyes respond much more weakly to violet than to blue.
a common mix-upWhat we see is exactly the light that arrives, with no filtering by our eyes.
4blue wins
Put the three pieces together: violet is scattered most, but the sun supplies less of it and our eyes detect it poorly. Multiply those effects and blue comes out on top. A small further effect: the atmosphere absorbs some of the sun's ultraviolet, the invisible light just beyond violet, but that is a minor factor here. The main story is the sun's spectrum and our eyes.
Strong scattering times weak supply times weak eyesight for violet leaves blue as the winner.
Name the two main reasons the sky looks blue instead of violet.
Show the answer
The sun emits less violet than blue, and our eyes are much less sensitive to violet.
So the blue sky is partly a fact about air and sunlight, and partly a fact about us: the color we call the sky's is the one our eyes handle best.