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Why is the sky blue, and sunsets red? (for a curious adult who has never studied physics). Cover, in this order: What is white light made of?; What does air do to a beam of light?; Why does blue light scatter more than red?; So why isn't the sky violet?; Why are sunsets red?; Why are clouds white? — page 2: Air Pushes Light Around
Air looks empty, but it isn't. Every breath holds around ten to the twenty-second molecules, a vast invisible crowd, and sunlight has to pass through all of them.
When light hits a molecule, the molecule absorbs some of it and re-emits it in many directions. This spreading-out of light is called scattering.
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.
The beam gets a little weaker going straight, but none of the light vanishes. It is redirected, so some of it reaches your eyes from parts of the sky away from the sun.
Sunlight scatters off air molecules, sending blue light toward you, so the sky glows. The Moon has no air, so its daytime sky stays black.
So the bright sky overhead is sunlight that air has redirected to you. Next we'll see why it favors one color.