Question

A parallel beam of white light falls on a combination of a concave and a convex lens, both of the same material. Their foacl lengths are 15cm and 30cm, respectively for the mean wavelength in white light. On the other side of the lens system, one sees colored patterns with violet color at the outer edge.

Answer 1

True. `(1)/(F)=(1)/(f_(1))+(1)/(f_(2))rArr (1)/(F)=(1)/(-15)+(1)/(30)`
`=(-2+1)/(30)rArr F=-30cm. `
This combination behaves as a concave lens of focal length 30 cm. Since `F_(v) lt F` , therefore one sees colored pattern with violet color at the outer edge. the statement is true.