Physics - Geometrical Optics Question with Solution | TestHub

For partial reflection, the final image should be$30 \mathrm{cm}$below water surface i.e.$20 \mathrm{cm}$below the mirror.

For the image $30 \mathrm{cm}$ below surface, its actual depth should be $\text{h}=30\left(\frac{4}{3}\right)=40cm$ below surface i.e. $30 \mathrm{cm}$ below the mirror

For mirror, $\text{u}=-\left(10+30\left(\frac{4}{3}\right)\right)$

$\Rightarrow \mathrm{u}=-50 \mathrm{cm}$ and $\mathrm{v}=30 \mathrm{cm}$

$\frac{1}{\mathrm{f}}=-\frac{1}{50}+\frac{1}{30}=\frac{\mathrm{-}3+5}{10}=\frac{2}{150}$

or $\mathrm{f}=75cm$ 

convex mirror of focal length $75 \mathrm{cm}$.
 

(diagram drawn was just for reference actually it should be a convex mirror but it was not known so we assumed concave but at final calculation, we found focal length +ve so it must be a convex mirror)