Question

A concave lens has focal length of 20 cm.At what distance should the object from the lens be placed so that it forms an image at 10 cm fromthe lens?

Answer 1

f=20

u=?

v=10

1/v_1/u=1/f-formula

1/10_1/u=1/20

1/u=1/20_(1/-10)

1/u=1+2/20

=3/20

=6.666

u= 6.666

Answer 2

focal length of concave lens , f = 20cm

image distance from the lens , v = -10cm

use formula,

$\quad\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

or, 1/20 = 1/-10 - 1/u

or, 1/u = 1/-10 - 1/20

or, 1/u = -( 2/20 + 1/20)

or, 1/u = -3/20

or, u = -20/3 = -6.67 cm

hence, object should placed 6.67 cm left side from the lens.