Question

Statement I If a convex lens of glass is immersed in water its power decreases.
Statement II In water, convex lens behaves as concave lens.

• A. Both Statement I and Statement II are correct and Statement II is the correct explanation of Statement I.
• B. Both Statement I and Statement II are correct but Statement II is not the correct explanation of Statement I.
• C. Statement I is correct but Statement II is incorrect.
• D. Statement I is incorrect but Statement II is correct.

Answer 1

Answer: (C)

From Lens maker's formula,
In air $\frac{1}{f_1}=\left(\frac{{\mu }_{\text{lens }}}{{\mu }_{\text{air }}}-1\right)\left[\frac{1}{R_1}-\frac{1}{R_2}\right]$
In water $\frac{1}{f_2}=\left(\frac{{\mu }_{\text{lens }}}{{\mu }_{\text{water }}}-1\right)\left[\frac{1}{R_1}-\frac{1}{R_2}\right]$
$\because \frac{{\mu }_{\text{lens }}(3/2)}{{\mu }_{\text{water }}(4/3)}<\frac{{\mu }_{\text{lens }}(3/2)}{{\mu }_{\text{air }}(1)}$
$\Rightarrow \frac{1}{f_2}$ or power is less in water but $f_2$ is positive,
hence it behaves as convex lens and not as concave lens.