Question

A tank of height 33.25 cm is completely filled with liquid $(\mu =1.33)$ An object is placed at the bottom of tank on the axis of concave mirror as shown in the figure. Image of the object is formed 25 cm below the surface of the liquid, then focal length of the mirror is

• A. 10 cm
• B. 15 cm
• C. 20 cm
• D. 25 cm

Answer 1

The correct option is C 20 cm

After first refraction from the surface of water, the image of object is at a depth of $\frac{33.25}{1.33}cm=25cm$.

Hence this image is at a distance of $25cm+15cm=40cm$ from the mirror.

Also the final image at $u=25cm$ depth can be retraced into air,

Depth of the final image retraced image$=\frac{25}{1.33}cm$

Hence the distance of this image from mirror$=\frac{25}{1.33}cm+15cm=v$

Using $\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$

we get

$f=18.3cm$