Question

A convex lens of focal length 20cm produces 3 times

Answer 1

i) For real image
Given , M = -3

m = v/u

-3 = v/u

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

1/20 = -1/3u -1/u

1/20 = -4/3u

u= -4 x 20/3 = - 80/3 cm

for virtual image

m= +3

m= v/u

3= v/u

v=3u

by lens formula

1/f = 1/v - 1/u

1/20 = 1/3u - 1/u

1/20= -2/3u

u= -2 x 20/3 = -40/3 cm.

therefore the two positions of the object are:

-80/3 cm and -40/3 cm

Answer 2

For real image

Given , M = -3

m = v/u

-3 = v/u

v = -3u .. (i)

by lens formula

1/f = 1/v -1/u

1/20 = -1/3u -1/u

1/20 = -4/3u

u= -4 x 20/3 = - 80/3 cm

for virtual image

m= +3

m= v/u

3= v/u

v=3u

by lens formula

1/f = 1/v - 1/u

1/20 = 1/3u - 1/u

1/20= -2/3u

u= -2 x 20/3 = -40/3 cm.

therefore the two positions of the object are:

-80/3 cm and -40/3 cm