# The Size of Image of an Object by a Convex Lens of Focal Length 20 Cm is Observed to Be Reduced to 1 3 Rd of Its Size . Find the Distance of the Object from the Optical Centre of the Lens . - Science

Numerical

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

#### Solution

Given :

f = 20  "cm" ; ν = 1/3 u  "cm" ; u = ?

we know that ,

1/f = 1/ν + 1/u

using sign convensions ,

1/f = -1/u - 1/ν

therefore 1/20 = -1/u - 3/4 = -4/u

therefore u = -4 xx 20 = -80  "cm"

therefore u = -80  "cm"

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