Explain the formation of a primary and a secondary rainbow after a rain shower, mentioning the optical phenomena involved in each case.
Answer
A primary rainbow is produced by refraction, one internal reflection and dispersion of sunlight in water droplets; a secondary rainbow involves two internal reflections, giving reversed colours.
Step-by-step solution
- Sunlight enters a spherical raindrop and is refracted, separating white light into its constituent colours (dispersion).
- Inside the drop, the light undergoes one internal reflection off the back surface and then emerges, refracting again as it leaves the drop; this sequence (refraction–reflection–refraction) creates the primary rainbow with red on the outer edge.
- For a secondary rainbow, the light experiences two internal reflections before exiting the droplet; the extra reflection reverses the order of colours, placing red on the inner edge and violet on the outer edge.
- The angular positions are about 42° for the primary and 51° for the secondary rainbow relative to the antisolar point.
Common mistake: Omitting the internal reflection step or confusing the order of colours in the secondary rainbow.
Concept tested: Rainbow formation and secondary rainbow
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Related questions
- 1 (a) Explain the formation of a rainbow in the atmosphere after a rain shower. State the three optical phenomena involved in sequence. (b) Why does the sun appear visible to us about 2 minutes before the actual sunrise?
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