MediumPhysicsHigh School (9–10)Optics › Prisms and Dispersion of LightConceptualBoard ExamAEEE (Amrita)AMUEEEAP EAPCETBCECECOMEDK UGET
Explain why a rainbow is a bow (part of a circle), and why red is on the outside and violet on the…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Explain why a rainbow is a bow (part of a circle), and why red is on the outside and violet on the inside. Why do we usually see only part of the circle?
Need a reminder? The key ideaShow
In each raindrop sunlight is refracted, reflected once at the back, and refracted again. Red comes back at about from the point opposite the Sun, violet at about , so red is on the outside. The point opposite the Sun is as far below the horizon as the Sun is above it. A fainter second bow, at about , has two reflections and its colours reversed.
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 circle. Red comes back to your eye from every drop that is from the point opposite the Sun. Those drops lie on a circle around that point.
Step 2. The order. Violet comes back at about : a smaller circle, inside the red one. The other colours lie between.
Step 3. Part of a circle. The point opposite the Sun is below the horizon, so the ground cuts off the lower part of each circle.
Answer: each colour comes from drops on a circle; red's circle is the largest, so red is outside; the ground hides the rest.
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