A lens forms a virtual image `4 cm` away from it when an object is placed `10 cm` away from it. The lens is a ….. Lens of focal length ……………

Options

A. concave, `6.67 cm`
B. concave, `2.86 cm`
C. convex, `2.86 cm`
D. may be concave or convex, `6.67 cm`.

Answer

A

Explanation

`(1)/(f)=(1)/(v)=(1)/(u)`
`(1)/(f)=(1)/(-4)-(1)/(-10)`
or `(1)/(f)=(1)/(10)-(1)/(4)` or `(1)/(f)=(2-5)/(20)=-(3)/(20)`
or `f=-(20)/(3)cm=-6.67cm`
The negative sign indicates that the lens is concave.