Question

A container is filled with water (μ= 1.33) up to a height of 33.25 cm. A concave mirror is placed 15 cm above the water level and the image of an object placed at the bottom is formed 25 cm below the water level. The focal length of the mirror is 

(a) 10 cm 

(b) 15 cm 

(c) 20 cm 

(d) 25 cm  

Answer 1

 Distance of object from mirror 

= 15+(33.25/1.33)= 40 cm 

distance of image from mirror 

= 15+(25/1.33) = 33.8 cm 

for the mirror, 1/v + 1/u = 1/f 

[1/(-33.8)] +[1/(-40)] =1/f 

f = -18.3 cm so most suitable answer is (c)