(a) To construct a ray diagram, we use two rays which are so chosen that it is easy to know their directions after reflection from the mirror. List two such rays and state the path of these rays after reflection in case of concave mirrors. Use these two rays and draw ray diagram to locate the image of an object placed between pole and focus of a concave mirror.
(b) A concave mirror produces three times magnified image on a screen. If the object is placed 20 cm in front of the mirror, how far is the screen from the object?

Explanation

(a) In fig. object AB is placed between pole P and focus F of concave mirror. A ray of light incident on the mirror in a direction parallel to principal axis passes through focus F on reflection from the mirror. Another ray, passing through centre of curvature (C) of mirror falls normally on the mirror, and retraces its path on reflection from the mirror. The image so formed is A'B', which is virtual, erect and magnified.
(b) Here, ` m - 3`, `u = -20 cm`
From ` m = v/u`
`-3 = v/(-20)`,`v = 60 cm`
Distance of screen from the object `= |v|- |u| = 60 - 20 = 40 cm`