Question

An object 1 cm high produces a real image 1.5 cm high, when placed at a distance of

15 cm from a concave mirror. Calculate : (i) the position of the image, (ii) focal length

of the concave mirror

Answer 1

Given :

Object height, hi = 1 cm

Image height, ho = - 1.5 cm ( given to be real )

Object distance, u = -15 cm

To Find :

Position of the image, v

Focal length of the mirror, f

magnification m = hi/h0 = -v/u

(-1.5)/1= - (v)/(- 15)

v = - 22.5 cm

the image is 22.5 cm in front of the mirror

1/v + 1/u = 1/f

1/-22.5 + 1/-15 =1/f

-2 -3 / 45 =1/f

-5/45=1/f

f= - 9

the focal length is 9 cm