Question

Plz answer.
Size of the image of an object by a concave lens of focal length 20cm is observed to be reduced to 1/3 rd of its size. Find the distance of the object from the lens.
2019 paper guys. I got ans as -40. Is it wrong.

Answer 1

yes , -40 is correct answer:

lets see the answer in cases!1

CASE 1:-

 m=-1/3(IN CASE OF CONCAVE MIRROR)

f = -20 cm

m = -1/3 = - v/u

v=u/3

1/f= 1/v +1/u

=> -1/20 = 3/u + 1/ u= 4/u

∴ u = -80 cm

CASE 2:-

M=+1/3 (IN CASE OF CONVEX MIRROR)

f=+20 cm

m=+1/3 =-v/u

v=-u/3

i/f=1/v+1/u

=> 1/20= -3/4 + 1/4 =-2/4

∴ u= -40 cm

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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