Question

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

Answer:

to get the size of image by convex lens of focal length 20cm so here you have to find the distance by using distance formula It's too easy that you can find easily

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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