Question

An object is placed at 20 cm from the pole of a concave mirror. It forms real image at a distance of 60 cm from the pole. Find the focal lenght of concave mirror.​ ​

Answer 1

Answer:

In the given problem,

Object distance, u = – 20 cm (using Cartesian sign convention)

Image distance, v = – 60 cm (using Cartesian sign convention for real image)

Focal length of the mirror is given by,

1/f = 1/u + 1/v

1/f = -1/20 + (–1/20) + (–1/60)

1/f = –(60 + 20)/60× 20 = –80/1200

f = –1200/80 = –15 cm.

Therefore, the focal length of the given concave mirror is 15 cm.

Answer 2

Answer:

the focal length of the mirror is -15 cm

Explanation:

u = -20 cm

v = -60 cm

by mirror formula,

1/f = 1/v + 1/u

1/f = 1/ -60 + (-1/20)

1/f = -1/60 - 1/20

1/f = -4/60

1/f = -1/15

f = -15 cm