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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

Answer in Brief
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 . 

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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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