Question

The size of image of an object by a convex lens of focal length 20cm is observed to be reduced to 1/3 of its size.find the distance of the object from the optical centre of the lens​

Answer 1

Magnification = 1/3

1/3  =  -v/u

u/3  =  v

mirror formula :

1/f =  1/v  +  1/u

sub v in formula :

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

1/-20  =  4/u  

1/u = 1/-80  

u=-80

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I hope it will helps you friend

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.