A concave mirror is placed at the bottom of an empty tank with face upwards and axis vertical. When sunlight falls normally on the mirror, it is focussed at distance of `32cm` from the mirror. If the tank filled with water `(mu=4//3)` up to a height of `20cm`, then the sunlight will now get focussed at

Answer

B

Explanation

(b) In first case, when sun is at infinity i.e,

`u=oo`
`rArr1/f=1/(-32)+1/(-oo)`
`thereforef=-32 cm`
When water is filled in the tank upto a height of 20 cm, the image formed by the mirror will act as virtual object O for water surface.
`therefore BI=BOxx3/4=12xx3/4=9 cm`