(a) Write the necessary conditions to obtain sustained interference fringes.

(b) In Young's double slit experiment, plot a graph showing the variation of fringe width versus the distance of the screen from the plane of the slits keeping other parameters same. What information can one obtain from the slope of the curve?

(c) What is the effect on the fringe width if the distance between the slits is reduced keeping other parameters same?

a) Necessary conditions for sustained interference fringes are:

i)     Two sources must be coherent. They should emit continuous light waves of same wavelength or frequency.

ii)    Two sources of light must be narrow.

iii)  Sources of light must be monochromatic.

b) Fringe width is given by:

                                       
where, D is the distance between the slit and the screen.

Fringe width and distance of screen from the slit is a linear graph.

The slope of the curve gives is  
That is, when fringe width varies linearly with distance of screen from the slits, the ratio of wavelength to distance between the slits remain constant. That is why it is advised to take wavelengths of incident light nearly equal to the width of the slit.

c) If the distance between the slits is reduced by keeping other parameters same. Then, fringe width would become broader.