1.

Find the power and type of the lens by which a person can see clearly the distant objects, if a person cannot see objects beyond 40cm

Answer»

`-2.5D` and concave LENS
`-2.5D` convex lens
`-3.5D` andconcave lens
`-3.5D` and convex lens

Solution :(a) Here, in this case the lens used by the person should form the image of DISTANT object of `40cm`
`:.u=oo,v=-40cm` and `1/f=1/v-1/u`
`1/f=1/(-40)-1/(-oo)`
`1/f=-1/40`
POWER `=1/f=-1/40=-2.5D`
Negative sign shows that lens used is concave lens.


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