An object is placed 1m in front of the curved surface of a plano-convex lens whose plane surface is silvered. A real image is formed in front of the lens at a distance of 120cm. Then, the focal length of the lens is

Options

A. 100cm
B. 120cm
C. 109.1cm
D. 110.0cm

Answer

c.

Explanation

`u=-100cm`
`v=-120cm`
`F=?`
`(1)/(F)+(1)/(u)+(1)/(v)=(1)/(-100)+(1)/(-120)`
`(1)/(F)=-(6+5)/(600)=-(11)/(600)`
Now, `(1)/(F)=(2)/(f_(1))+(1)/(f_(m))=(2)/(f_(1))`
or `f_(1)=2F`
or `f_(1)=2xx(600)/(11)`
[Negative sign is not to be used here.]
or `f_(1)=(1200)/(11) = 109.1cm`