Find the focal length of lens shown in the figure. Solve for three cases `n_(S) = 1.5, n_(S) = 2.0, n_(S) = 2.5`.

Answer

`360 cm, oo; -600 cm`

Explanation

`(1)/(f_(1)) = ((2)/(1.5) - 1) ((1)/(-60) - (1)/(-40)) rArr f_(1) = 360cm`
`(1)/(f_(2)) = ((2)/(2) - 1) ((1)/(-60) - (1)/(-40)) = 0 rArr f_(2) = oo`
`(1)/(f_(3)) = ((2)/(2.5) - 1) ((1)/(-60) - (1)/(-40)) rArr f_(3) = -600 cm`