The explanation, step by step
1. A blue sky
Look up on a clear day, and the whole sky glows blue. Why? The sunlight is white, yet the sky is blue. Let's find out where the blue comes from.
2. White light is all colours
Sunlight looks white, but it is really every colour mixed together. A glass prism spreads that white light out into a rainbow. Red light travels in long, lazy waves; blue light in short, quick ones.
3. Air scatters blue
High in the air, sunlight meets tiny molecules of nitrogen and oxygen. The long red waves mostly slip past them and keep going straight. But short blue waves hit the molecules and bounce off in every direction.
4. Blue from every direction
So scattered blue light reaches your eyes from every part of the sky. That's why the whole sky looks blue, not just the part near the Sun! At sunset, light crosses so much air that the blue is scattered away, leaving reds. Same sunlight, same air: tiny molecules paint the sky blue by day.
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