Explanation
Power of a lens =\dfrac{1}{f}
Where f is the focal length of the lens.
So, by lens maker formula
\dfrac{1}{f}=\left ( \dfrac{\mu _{2}}{\mu _{1}}-1 \right )\left ( \dfrac{1}{R_{1}}-\dfrac{1}{R_{2}} \right )
As lens is dipped in water, \mu of the medium is increased from 1 to \dfrac{4}{3} (i.e. refractive index of water), so, \dfrac{\mu_{2}}{\mu_{1}} term decreases, so, \left ( \dfrac{\mu_{2}}{\mu_{1}}-1 \right ) term decrease which leads to decrease of \dfrac{1}{f}. so, power \left ( =\dfrac{1}{f} \right ) decreases.
Please disable the adBlock and continue. Thank you.