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 . 

Answer 1

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"`