Question

A convex lens is immersed in a liquid, whose refractive index is equal to the refractive index of the material of the lens. Then its focal length will

• A. Decrease
• B. Become zero
• C. Become infinite
• D. Increase

Answer 1

The correct option is C Become infinite

Given, ${}_a{\mu }_g{=}_a{\mu }_l$
The focal length of convex lens in liquid f is given by
$\frac{1}{f}(\frac{{}_a{\mu }_g}{{}_a{\mu }_l}-1)(\frac{1}{R_1}-\frac{1}{R_2})$
$\frac{1}{f}=(1-1)(\frac{1}{R_1}=\frac{1}{R_2})$
$\frac{1}{f}=0$
or $f=\infty$
Its focal length will become infinite.