A container is filled with water `(mu=1.33)` upto a height of 33.25 cm. A concave mirror is placed 15cm above the water level and the image of an object placed at the bottom is formed 25 cm below the water level. Focal length of the mirror is

Options

A. 15 cm
B. 20 cm
C. `-18.31 cm`
D. 10 cm

Answer

C

Explanation

The image I` for first refraction (i.e., when the ray comes out of liquid) is at a depth of
`=(33.25)/(1.33)=25 cm` `[:'Apparent depth=(Real depth)/(mu)]`
Now, reflection will occur at concave mirror. For this I`
`:' u=-(15+25)=-40 cm`
and `v=-[15+(25)/(1.33)]`
Where `(25)/(1.33)` is the real depth of the image.
Using mirror formula we get
`(1)/(f)=(1)/(v)=(1)/(u)` `f=-18.31 cm`