A biconvex lens has a focal length of 10 cm in air. If it is immersed in a liquid of refractive index 1.5, and the lens material has a refractive index of 1.5, what will be its new focal length?

C. Infinity — Using the lens maker's formula, the focal length depends on the ratio of the refractive index of the lens to the surrounding medium. If the indices are equal, the lens behaves like a plane glass plate.

JEE Advanced · PhysicsMedium

$A biconvex lens has a focal length in air (). If it is immersed in a liquid of refractive index , what will be its new focal length ?$

A. $10 cm$
B. $20 cm$
C. $Infinity$
D. $Zero$

Show correct answer & step-by-step solution

Correct answer: C — $Infinity$

Solution

\frac{1}{f} = (\frac{μ _{g}}{μ _{m}} - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}}) . Since μ_{g} = μ_{m}, the term (\frac{μ _{g}}{μ _{m}} - 1) = 0, so f = \infty .

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A biconvex lens has a focal length fa​=10 cm in air (μg​=1.5). If it is immersed in a liquid of refractive index μl​=1.5, what will be its new focal length fl​?

Solution

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$A biconvex lens has a focal length in air (). If it is immersed in a liquid of refractive index , what will be its new focal length ?$