A convex lens A of focal length 20cm and a concave lens B of focal length 5cm are kept along the same axis with a distance d between them. I a parallel lbeam of leight falling on A leaved B as a parallel beam, then d is equal to `"____________"`cm.

Explanation

Since the beam leaves B as a parallel beam, therefore the effective focal length of the two lenses combination is `prop`. For two lenses placed at some distance d apart, the focal length of the combination is given by the formula
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))-(d)/(f_(1)f_(2))`
`(1)/(oo)=(1)/(+20)+(1)/((-50))-(d)/((+20)(-5))rArrd=+15cm`