A slide projector has to project a 35 mm slide `(35 mm xx 23 mm)` on a 2 m xx 2m screen at a distance of 10 m from the lens. What should be the focal length of the lens in the projector ?

Answer

A, B, C

Explanation

Magnification
`(m)=(2xx10^3)/35=v/u`
`:.u=35/(2x10^3)`
`v=35/(2xx10^3)xx10xx10^3`
`=175mm`
From lens Formula
`1/v-1/u=1/f`
`rarr 1/10+1/0.175=1/f`
`rarr 10175/1750=1/f`
`rarr f=1750/10175, mu=17.2cm`