Question

An object 4 cm in size is placed at 20 cm in front of a concave mirror of focal 10 cm.At what distance from the mirror should a screen be placed in order to obtain a sharp image

Answer 1

Answer

The screen should be placed at a distance of 20cm

Explanation

Calculate image distance

Here object distance u=-20cm

focal length = 10 cm

From this we get image is placed at center of curvature

Let image distance be 'v'

We have mirror formula,

1/v + 1/u = 1/f

1/v =1/f - 1/u

1/v = [1/(-10) -  1/(-20)] =(-1)/20

Therefore v =-20

The screen should be placed at a distance of 20cm

Answer 2

ht of the object = 4 cm

object distance = 20 cm

focal length = 10 cm

By using mirror formula we have

1/f = 1/u + 1/v

1/-10 = 1/-20 + 1/v

or v = -20 cm

magnification = -v/u

h1/h0 = -v/u

h1/4 = -(-20)/20

or h1 = 4 cm