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Question
Answer the following question in detail.
Explain the formation of a primary rainbow. For which angular range with the horizontal is it visible?
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Solution
- A ray AB incident from Sun (white light) strikes the upper portion of a water drop at an incident angle i.
- On entering into the water, it deviates and disperses into constituent colours. The figure shows the extreme colours (violet and red).
Formation of primary rainbow - Refracted rays BV and BR strike the opposite inner surface of water drop and suffer internal reflection.
- These reflected rays finally emerge from V′ and R′ and can be seen by an observer on the ground.
- For the observer, they appear to be coming from the opposite side of the Sun.
- Minimum deviation rays of red and violet colour are inclined to the ground level at θR = 42.8° ≈ 43° and θV = 40.8 ≈ 41° respectively. As a result, in the rainbow, the red is above and violet is below.
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