A biconvex lens of refractive index 1.5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index 1.6 will be:

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Solution

Q.
A biconvex lens of refractive index 1.5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index 1.6 will be: [2024]

1 -16 cm

2 -160 cm

3 +160 cm

4 +16 cm

Ans.
(2)

$μ_{0} = 1.5, μ_{liq} = 1.6 a n d f_{a i r} = 20 c m$

$A s \frac{f_{l i q}}{f_{a i r}} = \frac{(μ_{0} - 1) μ_{l i q}}{(μ_{0} - μ_{l i q})} \Rightarrow \frac{f_{l i q}}{20} = \frac{(1.5 - 1) 1.6}{(1.5 - 1.6)}$

$\Rightarrow f_{l i q} = - 160 c m$

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