First slide

The lens

Question

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

Moderate

A

25 D
B

– 25 D
C

1 D
D

None of these

Solution

$\frac{f_{l}}{f_{a}} = \frac{(_{a} μ_{g} - 1)}{(_{l} μ_{g} - 1)} \Rightarrow \frac{f_{l}}{f_{a}} = \frac{{aμ}_{g} - 1}{\frac{μ_{g}}{μ_{l}} - 1} = \frac{1 .5 - 1}{\frac{1 .5}{1.6} - 1} = \frac{0 .5 \times 1 .6}{- 0 .1} = - 8 \Rightarrow P_{l} = \frac{P_{a}}{8} = \frac{5}{8}$

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