Question

size of image of an object by concave lens of focal length 20cm is observed to be reduced to one third of its size find the distance of object from the lens

Answer 1

Answer kitna h iska!!

Answer 2

hi mate,

Solution: object distance from the optical centre of the lens is 40cm.

explanation : focal length of concave 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, 2/u = 1/-20 cm

or, u = -40 cm

hence, object is placed 40cm from the pole of concave lens.

i hope it helps you.......

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