Question

A convex lens of focal length 30cm produces a real image 3times magnified. Find the object distance.

Answer 1

For real image
Given , M = -3

m = v/u

-3 = v/u

v = -3u ...................... (i)
by lens formula
1/f = 1/v - 1/u

1/30 = -1/3u - 1/u

1/30 = -4/3u

u= -4 x 30/3 = - 40 cm

for virtual image

m= +3

m= v/u

3= v/u

v=3u

by lens formula

1/f = 1/v - 1/u

1/30 = 1/3u - 1/u

1/30= -2/3u

u= -2 x 30/3 = -20cm.

therefore the two positions of the object are:

-40 cm and -20 cm