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Question
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 .
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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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