Sunlight is made of many colors. When it travels through the air, tiny parts of the air scatter blue light much more than red light.

So blue light comes from all over the sky, and the sky looks blue. In the evening, sunlight travels a longer way through the air. Most of the blue light is lost on the way, so the sunset looks red.

Sunlight contains all colors, and each color has a different wavelength. In 1871, the British physicist Lord Rayleigh showed that tiny particles in the air scatter light with shorter wavelengths much more strongly. Blue light is scattered about five to six times more than red light, so blue light reaches our eyes from every part of the sky.

Violet light is scattered even more, but sunlight contains less violet, and our eyes are not very sensitive to it. At sunset, sunlight passes through much more air, so most blue light is scattered away before it reaches us, leaving red and orange. Clouds look white because their water drops scatter all colors equally.

In 1871 Lord Rayleigh worked out why the sky is blue. Particles much smaller than a wavelength of light, such as nitrogen and oxygen molecules, scatter light in inverse proportion to the fourth power of its wavelength.

So blue light, at around 450 nanometers, is scattered roughly five to six times more than red light at around 700. By that rule alone, the sky ought to look violet, because violet is scattered even more. It doesn't, for two main reasons.

Sunlight contains relatively little violet, and our cone cells respond only weakly to violet. The brain reads the resulting mix as pale blue. At sunset, light crosses a much longer stretch of atmosphere, so the blue is scattered out along the way and reddish light is left. Scattering cannot do all the work at twilight, though.

Calculations suggest that the sky overhead would turn yellowish if ozone did not absorb some orange and red light. Clouds look white because their droplets are far larger and scatter all wavelengths about equally. On dusty Mars the pattern flips. Sunsets there look bluish, because fine dust scatters blue light forward, toward the observer.