Q.
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.
Ray Optics and Optical Instruments
Solution:
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.
