Question

A 2 cm high candle flame is placed at a distance of 80 cm from a white screen. On placing a convex lens exactly at the mid point of the candle and the screen, a distant image of the flame is seen on the screen. What is the focal length of the lens and the size of the candle flame image formed?

Answer 1

$h=2cm,$

$u=-40cm,$

$v=40cm$

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{40}-\frac{1}{-40}=\frac{1}{f}$

$f=20m$

$m=\frac{v}{u}=\frac{40}{-40}=-1$

The size of image is same as the object i.e. it is of 2 cm but inverted.