# When a pencil of white light, parallel to the principal axis, is refracted by a converging lens, the rays converge on the principal axis, the focus being spread out over short range along the principal axis. If a screen is put perpendicular to the principal axis at the farthest focus, the image on the screen is 1)a sharp violet point 2)a sharp red point 3)a circular dispersed patch, violet at the centre and red at the border 4)a circular dispersed patch, violet at the border and red at the centre Since the focal length for red light is more, the violet rays will come to focus at a nearer point than red rays. So when the screen is placed at the farthest focus(i.e. at the focal point of red rays), there will be a a sharp red point. But the answer given in my book is (4) a circular dispersed patch, violet at the border and red at the centre. Could someone please the explain the reason for this?
The red light is focussed to a point, as you say. The blue light is focused in front of the screen, then spreads out a little. This results in a circle of violet light around the red spot, because the lens is circular. Between the violet circle and the red spot are all the other colours of the rainbow.
ROYGBIV That's the order of light in a rainbow. Red at one end, violet at the other. So if the Red is in focus forming the point in the center, the violet is the most out of focus and so forms the outer ring.
