Question

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

• A. Become zero
• B. Become infinite
• C. Become small, but non-zero
• D. Remain unchanged

Answer 1

The correct option is B Become infinite

We have the lens makers formula

$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$

Here $\mu =\frac{{\mu }_{convexlens}}{{\mu }_{liquid}}=1,\therefore \frac{1}{f}=0$

$f=\infty$

Focal length of the lens inside the liquid is infinite.