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

Answer: (B)

become infinite

Answer 2

Use $\frac{1}{ f }=(\mu-1)\left(1 / R _{1}-1 / R _{2}\right)$
Here $\mu= \frac{\mu_{\text {convexlens }}}{\mu_{\text {liquid }}}=1$
$\therefore f =\infty$