A LASER source placed between two concave mirrors sends light rays as shown in the figure. If PQ and RS are parallel to the principal axis, determine the focal lengths and radius of curvatures of both the mirrors.

Explanation

As the light ray after reflection from the first mirror travels parallel to the principal axis, it is placed at the principal focus of first mirror. The reflected ray from the second mirror travel through the focus of the first mirror. Therefore, the source is also placed at the focus of second mirror.
So, focal length f= 10 cm and radius of curvature R `=2xxf=2xx10=20cm`