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. convex mirror of focal length 10 cm
B. concave mirror of focal length 40 cm
C. concave mirror of focal length 60 cm
D. concave mirror of focal length 10 cm

Answer

A

Explanation

`(1)/(F)=(1)/(f_1)+(1)/(f_m)+(1)/(f_1)`
`=(2)/(f_1)+(1)/(f_m)=(2)/(-20)+(1)/(infty)=-(1)/(10)`
`F=-10cm`
This combination will behave like a convex mirror of focal length of 10 cm.