A converging lens is to projected image of a lamp 4 times the size of the lamp on a wall at a distance of `10m` from the lamp. The focal length of the lens is

Options

A. `1.6 m`
B. `2.67m`
C. `4.4m`
D. `-1.6m`

Answer

A

Explanation

`m=(v)/(u)`
`-4=(v)/(u)` or `v=-4v`
Now, `v-u=10`or `-5u=10`
or`u=-2m`
`v=8cm`
Again, `(1)/(f)=(1)/(v)-(1)/(u)`
or `(1)/(f)=(1)/(8)-(1)/(-2) `or `(1)/(f)=(1)/(8)+(1)/(2)=(1+4)/(8)=(5)/(8)`
or `f=(8)/(5)m=1.6m`