A convex lens and a convex mirror are placed at a separation fo `15cm`. The focal length of the lens is `25cm` and radius fo curvalture fo the mirror is `80 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 refracted by the lens, comes out parallel to the principle 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`