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 distance of object from optical center of lens​

Answer 1

30.... something should be its 19 .6 as its answer

Answer 2

Lens formula is given by

1/f = 1/v - 1/u

v  is the distance of image from optical centre of the lens.

u is the distance of object from optical centre of the lens.

f is the focal length of the lens.  

here,

Height of image / height of object = v/u

Let height of object be H.

It is given that the size of image is 1/3 of object size.  

Therefore,

(H / 3) / H = v/u

v/u = 1/3

v = u/3

focal length f = 20cm  

Substitute the values in the formula.

1/20 = 3/u - 1/u

1/20 = 2/u

u = 40cm

Answer :  

Distance of object from optical center of lens is 40 cm.