Question

A convex lens of focal length $f$ made of glass of refractive index $1.5$ is immersed in water of refractive index $4/3$. Then focal length:

• A. $f$
• B. Greater than $f$
• C. Less than $f$
• D. $-f$

Answer 1

Answer: (B)

Greater than $f$

Lensmaker equation gives the relation for focal length as

$\frac{1}{f}=(\frac{n_1}{n_2}-1)K_{\text{shape}}$

Given that, in air, the focal length of the lens is $f$

$\Rightarrow K_{\text{shape}}=\frac{1}{2f}$

Focal length in water is given by

$\frac{1}{f_{\text{water}}}=(\frac{n_1}{n_2}-1)K_{\text{shape}}=(\frac{3/2}{4/3}-1)\frac{1}{2f}\Rightarrow f_{\text{water}}=16f$