Question

The size 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 optical centre of the lens.

Answer 1

Answer:

The distance of the object from the optical centre of the lens will be 20 cms

Explanation:

Given data,

Focal length = 20 cm

magnification = 1/3

We know that magnification is given by the formula,

m = v/u

where v = image distance

u = object distance

=> 1/3 = v/u

=> v = u/3

From the lens formula for concave lens

1/v - 1/u = 1/f

=> 1/(u/3) - 1/u = 1/f

=> 3/u - 1/u = 1/f

=> 2/u = 1/f

=> u = 2f = 2 x 20 = 40 cm

Hence the distance of the object from the optical centre of the lens will be 20 cms

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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