alleryA lens is held in water. Its focal length compared with that in air:
AIncreases
BDecreases
CBecomes zero
DDoes not change
Answer & Solution
Correct answer: A. Increases
1. Refractive index of water (1.33) is close to that of glass (1.50).
2. Lens-maker's equation has (n_lens / n_medium - 1) factor.
3. In water this factor is smaller than in air.
4. So the lens bends rays less in water.
5. Focal length therefore increases when the lens is in water.
_Source: Selina Concise Physics ICSE Class 10, Ch 5 'Refraction through a Lens' (aplustopper.com extract)_
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