What is the effect on the interference fringes in a Young’s double-slit experiment due to each of the following operations:
(a) the screen is moved away from the plane of the slits,
(b) the (monochromatic) source is replaced by another (monochromatic) source of shorter wavelength,
(c) the separation between the two slits is increased
(d) the source slit is moved closer to the double-slit plane,
(e) the width of the source slit is increased
(f ) the monochromatic source is replaced by a source of white light? ( In each operation, take all parameters, other than the one specified, to remain unchanged.)

Explanation

Let s be the size of the source and S its distance from the plane of the two slits. For interference fringes to be seen, the condition `s"/"S lt lambda"/"d` should be satisfied, otherwise, interference patterns produced by different parts of the source overlap and no fringes are seen. Thus, as S decreases (i.e., the source slit is brought closer), the interference pattern gets less and less sharp, and when the source is brought too close for this condition to be valid, the fringes disappear. Till this happens, the fringe separation remains fixed.