A converging lens and a diverging mirror are placed at a separation of 15 cm. The focal length of the lens is 25 cm and that of the mirror is 40 cm. Where should a point source be placed between the lens and the mirror so that the light, after getting reflected by the mirror and then getting transmitted by the lens, comes out parallel to the principal axis ?

Answer

`(5)/(3) cm` from the lens

Explanation

For final light ray to be parallel ot the axis of the lens, the image formed by the mirror should be at the focous of the lens.
By mirror formula `(1)/(v) + (1)/(u) = (1)/(f), v = +10 cm, f = 40cm`
on solving `u = -(40)/(3) cm` so distance of object form lens `= 15 - (40)/(3) = (5)/(3) cm`