A concave lens of focal length 20 cm placed in contact with ah plane mirror acts as a convex mirror of focal length

Options

A. 10 cm
B. 40 cm
C. 60 cm
D. 20 cm

Answer

A

Explanation

For lens, mirror combination, combined focal length is given by
`(1)/(F)=(2)/(f)+(1)/(f_(m))`
Here, `f_(m)=oo`
`therefore(1)/(F)=(2)/(f)`
or `F=(f)/(2)=(20)/(2)=10cm`