Question

size of the image of an object by a concave lens of focal length 20cm is observed to be reduced to 1/3rd of its size. find the distance of the object from the lens.


someoe plz give the rite answer wid proper steps.
help me...!!!!!!

Answer 1

m = 1/3

v/u = 1/3

3v = u

f = -20cm

by lens formula

1/v - 1/u = -1/20

1/v - 1/3v = -1/20

2/3v = -1/20

-40/3 = v

v = 13.33 cm

u = 13.33 * 3

= 39.999 cm

Answer 2

hi mate,

Solution: object distance from the optical centre of the lens is 80cm.

explanation : focal length of convex lens , f = +20cm

a/c to question,

size of image = 1/3 × size of object.

so, magnification , m = -1/3

we know, magnification of lens , m = v/u

so, v/u = -1/3

so, v = -u/3 ........(1)

now applying lens maker formula,

1/v - 1/u = 1/f

or, 1/(-u/3) - 1/u = 1/20

or, -3/u - 1/u = 1/20

or, u = -80 cm

hence, object is placed 80cm from the pole of convex lens.

i hope it helps you.