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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 3: Blue Scatters Most
Each color is a wave. Red has long, stretched-out waves. Blue has short, tightly packed ones. That length is the only difference we need.
The light wave shakes the molecule like a cork bobbing on water. The jiggling molecule then sends light back out in every direction. That's scattering.
Shorter waves wiggle faster, and the scattering climbs steeply. Halve the wavelength and you get sixteen times more. Blue ends up scattered five or six times more than red.
Sunlight enters white, but air bounces blue light sideways far more than red. That blue then reaches your eyes from all directions, so the whole sky looks blue.
The sky is blue because air scatters short waves far more than long ones, sending blue light to you from every direction.