Light getting bounced
Why Is the Sky Blue?
Air scatters blue light in every direction, so blue arrives from the whole sky.
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Step 1 of 5
Sunlight is every colour mixed together.
A prism does not colour the light. It only sorts it. It looks white, but a prism splits it into red, orange, yellow, green, blue and violet. All of them arrive from the Sun together.
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Step 2 of 5
Colours are waves of different sizes.
Red light has long, lazy waves. Blue light has short, tight ones. That size difference is what makes the sky blue.
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Step 3 of 5
Air molecules bounce the short waves hardest.
Nitrogen and oxygen molecules are just the right size to knock blue light sideways — about ten times more of it than red. Red light mostly sails straight past them.
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Step 4 of 5
So blue comes at you from the whole sky.
Which is why the sky glows blue in the places the Sun is not. The scattered blue has bounced around and now arrives from every patch of sky, not just from the Sun's direction. That is what you see overhead.
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Step 5 of 5
At sunset the light travels much further.
Low sunlight crosses far more air, so nearly all the blue is scattered away before it reaches you. What is left is red and orange.
Violet scatters even more than blue, but our eyes are far less sensitive to it — so we see blue.
The short version
Air molecules are the right size to knock short blue waves sideways, about ten times more than red, so blue reaches you from every patch of sky. The same scattering, with more air to cross, is what makes a sunset red.
Try it yourself
Shine a torch through a glass of water with a drop of milk in it. From the side it looks bluish; from the end, orange.