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 Fig. Image of the object is formed at 25 cm below the surface of the liquid. 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

Apparent depth $\frac{33.25}{1.33}=25cm$ when object is at $2f$

Image is formed at $2f$.

$\therefore$ $2f=15+25=40cmandf=20cm$