Question

A concave mirror and a convex lens both have same focal length in air. When they are submerged in glycerine, the focal length :

• A. Of both still remains unchanged
• B. Of the mirror remains unchanged while that of the lens increases
• C. Of the mirror remains unchanged while that of the lens decreases
• D. Of both may increase or decrease

Answer 1

The correct option is B Of the mirror remains unchanged while that of the lens increases

We know,

$\frac{1}{f_L}=(\frac{{\mu }_L}{{\mu }_M}-1)(\frac{1}{R_1}-\frac{1}{R_2})$

And

$\frac{1}{f_M}=\frac{2}{R}$

It is clear that focal length for mirror depends only on Radius of curvature,

But focal length of lens also depends upon the refractive index

of the surrounding.

Hence, Option $\text{B}$ is the correct answer