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

Explanation

`(1)/(F)=(1)/(f_1)+(1)/(f_2)`
`implies (1)/(F)=(1)(-15)+(1)/(-30)=(-2+1)/(30)` `Implies F=-30cm.
This combination behaves as a concave lens of focal length 30 cm.
Since `F_vltF_r`.
:' One sees coloured pattern with violet colour at the outer edge.