Question
A rainbow is an example of dispersion of sunlight. Justify this statement by explaining, with a neat labelled diagram, the formation of a rainbow in the sky. List two essential conditions for observing a rainbow.
✓
Answer
- A rainbow is a natural spectrum appearing in the sky after a rain shower. [figure]
- Rainbow is caused by dispersion of sunlight on refraction by tiny water droplets, present in the atmosphere.
- It is always formed in the direction opposite to that of the Sun.
- So the rainbow is seen when the Sun is behind the observer.
- Here, water droplets act like small prisms.
- They first refract and disperse the incident sunlight, then reflect it internally (not necessarily total internal reflection) and finally refract it again while it comes out of the raindrop.
[Formation of the main or primary rainbow]
- This phenomenon is shown using schematic diagram in the figure, using only one water drop out of many water drops.
- From the figure one can say that, the water drop causes internal reflection once and refraction twice of the incident ray of sunlight.
- Due to dispersion of light and internal reflection, different colours reach the observer’s eye.
- Conditions for observing a rainbow:
- $A.$ after rainfall/ at a water fountain.
- $B.$ The Sun should be behind the observer.
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