Question

Assertion : When a convex lens $\left(\mu_{ g }=3 / 2\right)$ of focal length $f$ is dipped in water $\left(\mu_w=\frac{4}{3}\right)$, its focal length becomes $4 f$.
Reason : The focal length of convex lens is always half of its radius of curvature.

• A. If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
• B. If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
• C. If the Assertion is correct but Reason is incorrect.
• D. If the Assertion is incorrect and Reason is correct.

Answer 1

Answer: (C)

$f_w=f \frac{{ }_a \mu_g-1}{\left(\frac{{ }_a \mu_g}{{ }_a \mu_w}-1\right)}=f \frac{\left(\frac{3}{2}-1\right)}{\left(\frac{3 / 2}{4 / 3}-1\right)}=4 f$