Question

A convex lens of glass ($\mu =1.5$) is immersed in water. Compared to its power in air, its power in water-

• A. increases
• B. decreases
• C. remain same
• D. nothing can be predicted

Answer 1

The correct option is B decreases

From lens makers formula we have

$\frac{1}{f}=\left(\frac{{\mu }_{lens}}{{\mu }_{medium}}-1\right)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$

It is given that ${\mu }_a=1$ and ${\mu }_w=\frac{4}{3}=1.33$.

Hence ${\mu }_a<{\mu }_w$

$\frac{{\mu }_g}{{\mu }_a}>\frac{{\mu }_g}{{\mu }_w}$

$\therefore \frac{1}{f_a}>\frac{1}{f_w}$

Or power $P_a>P_w$.

Hence the correct answer is option (B) Decreases.