Question

Radius of curvature of concave mirror is 40cm and the size of image is twice as that of object, then the object distance is(a) 60cm(b) 20cm(c) 40cm(d) 30cm

Answer 1

answer : option (d) 30cm

explanation : radius of curvature of concave mirror, R = -40cm

so, focal length of concave mirror,f = -20cm

size of image is twice as that of object.

so, magnification , m = size of image/size of object
= 2/1 = 2

now, magnification , m = -v/(-u) = 2
v = 2u ......(1)

use formula, 1/v + 1/u = 1/f

or, 1/(2u) + 1/u = 1/-20 [ from equation (1) ]

or, 3/2u = 1/-20

hence, u = -30cm

Answer 2

Answer:

30cm is the answer

hope it helps