First slide

The lens

Question

A double convex thin lens made of glass of refractive index 1.6 has radii of curvature 15 cm each. The focal length of this lens when immersed in a liquid of refractive index 1.63 is

Moderate

A

– 407 cm
B

250 cm
C

125 cm
D

25 cm

Solution

$\frac{f_{l}}{f_{a}} = \frac{({aμ}_{g} - 1)}{(μ_{g} - 1)}; f_{a} = \frac{R}{2 (μ_{g} - 1)} = \frac{15}{2 (1 .6 - 1)} = 12 .5 \Rightarrow \frac{f_{l}}{12 .5} = \frac{(1 .6 - 1)}{(\frac{1 .6}{1 .63} - 1)} \Rightarrow f_{l} = - 407 .5 cm$

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