1.

An image 2/3 of the size of the object is formed by a convex lens at a distance of 12 cm from it. Find the focal length of the lens.

Answer»

Solution :The IMAGE formed is REAL (CONVEX lens forms `m=-2/3,v=+12cm,m=v/u-2/3`
`-2u=36`
`u=-18cm`
`1/f=1/v-1/u`
`1/f=1/12-1/(-18)`
`=(3+2)/36`
`f=36/5=7.2cm`


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