What is the focal length if plane surface is silvered in previous illustration.

Explanation

`(1)/(f_(l)) = (mu - 1) ((1)/(R) - (1)/(oo)) = (mu - 1)/(R)`
`(1)/(f_(m)) = (1)/(oo) = 0`
Equivalent focal length (F)
`(1)/(F) = (-2)/(f_(l)) + (1)/(f_(m)) = (-2(mu - 1))/(R)`
`rArr F = (R)/(-2(mu - 1))`