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A thin glass (refractive index 1.5) lens has optical power of −5 D in air. Its optical power in a liquid medium with refractive index 1.6 will be

1. −1 D
2. 1 D
3. −25 D
4. 25 D

Correct answer: 1 D

Solution

P_liquid = P_air * (n_g/n_l - 1)/(n_g - 1) = -5 * (1.5/1.6 - 1)/(1.5 - 1) = +0.625 D. The lens becomes weakly converging (positive power), nearest to 1 D.

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