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Air Pushes Light Around
Sunlight crossing the air is not left alone: tiny molecules knock some of it off its straight path and send it out in all directions.
5 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1crowded air
Air looks like nothing, but it is a crowd of molecules, mostly nitrogen and oxygen, far too small to see. A single breath holds roughly 10^22 of them, an enormous number. Light passing through air is passing through this crowd.
Empty-looking air is really a vast crowd of tiny molecules.
a common mix-upClear air is not 'nothing'; it is full of molecules that can interact with light.
If air were truly empty, what would a beam of sunlight do?
Show the answer
Travel straight through unchanged.
2scattering
When a beam meets a molecule, the molecule absorbs a little of that light's energy and immediately re-emits it. The re-emitted light does not keep the original direction; it spreads outward, in many directions. This redirecting is called scattering.
Scattering means a molecule takes in light and sends it back out in new directions.
a common mix-upScattering is not the light bouncing off like a ball; the molecule absorbs and re-emits it, sending it out in new directions.
3scattering
When light hits a molecule, the molecule absorbs some of it and sprays it back out in many directions. This spreading-out of light is called scattering.
4not lost
Because each molecule redirects a little light, the beam loses some of its straight-ahead brightness. That light has not vanished; it has been sent sideways, upward, backward. So some of the sunlight now reaches your eyes from directions other than the sun itself.
Scattered light isn't lost; it's redirected, so it can reach you from anywhere in the sky.
a common mix-upScattering does not destroy light; it only changes where it travels.
When a molecule scatters light out of the beam, where does that light go?
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It continues in a different direction, so it can reach you from other parts of the sky.
5Earth vs Moon
This is why the sky glows even where the sun isn't. Looking away from the sun, you see no direct sunlight, yet the sky is bright because air molecules are redirecting sunlight toward you. Without air, as on the Moon, the sky is black even by day. Which colors get scattered most is the next page's story.
The daytime sky glows because air redirects sunlight to your eyes; no air, no glow.
So the bright sky overhead is sunlight that air has redirected to you. Next we'll see why it favors one color.