Question:
Rays from sun converge at a point 15 cm in front of a concave mirror. Where should an object be placed so that size of its image is equal to the size of the object ?
(a) 15 cm in front of the mirror
(b) 30 cm in front of the mirror
(c) between 15 cm and 30 cm in front of the mirror.
(d) more than 30 cm in front of the mirror.
Solution:
(b).
Explanation : Here, Focal length of concave mirror, f = -15 cm Radius of curvature of the mirror, R = 2f = -30 cm.
In case of concave mirror, size of image is equal to the size of the object if the object is placed at the centre of curvature (see figure 20). The
distance of centre of curvature from the mirror = radius of curvature of the mirror.