Question

If the source of light used in a Young's double slit experiment is changed from red to violet,
(a) the fringes will become brighter
(b) consecutive fringes will come closer
(c) the intensity of minima will increase
(d) the central bright fringe will become a dark fringe

Answer 1

(b) consecutive fringes will come closer

EXPLANATION:   

The options (a), (c) and (d) are related to the intensity changes. Since the intensity of light incident on the slits is not changed hence there will be no change in the intensity of the fringes. These options are not true.           

The fringe width is given as w = Dλ/d.  In this experiment D and d are not changed only λ is changed. Since the violet has smaller λ than the red light, so the fringe width will decrease. Hence the consecutive fringes will come closer. Option (b) is true.