Question

The focal length of a concave mirror is 50cm. Where an object be placed so that its image is two times and inverted(a) 75 cm(b) 72 cm(c) 63 cm
(d) 50 cm

Answer 1

answer : option (a) 75cm

explanation : focal length of concave mirror, f = -50cm

Let the size of object is h

then, size of image = -2h [ negative because image is inverted ]

now, magnification , m = height of image/height of object = -v/u

or, -2h/h = -v/u

or, v = 2u .....(1)

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

or, 1/2u + 1/u = 1/-50

or, 3/2u = -1/50

hence, u = -75cm

Answer 2

Answer:

Given,

Focal length, f=50cm

Magnification, m=2

m=uv=2

v=2u

From mirror formula,

f1=v1+u1

f1=2u1+u1

u=23f=23×50=75 cm

Object Is placed at 75cm from the mirror.