The blue of the sky and the red of a sunset are both caused by scattering…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The blue of the sky and the red of a sunset are both caused by scattering. Explain how one effect gives both colours.
Need a reminder? The key ideaShow
The sky is blue because air scatters blue toward us from every direction. Near the horizon sunlight crosses much more air, about times as much for a Sun degrees high, so most of the blue is scattered away and the Sun looks red.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The sky. Air molecules scatter blue light much more than red, in every direction. Look away from the Sun, and the scattered blue reaches you from all over the sky.
Step 2. The sunset. Near the horizon, sunlight crosses a long path of air. Most of its blue is scattered out of the beam on the way.
Step 3. One effect, two colours. The scattered light, seen from the side, is blue; the light left in the beam, seen straight on, is red.
Answer: scattering sends blue out of the sunbeam in all directions: we see the blue in the sky and the red that is left in the beam.
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