PLZ DERIVE THE EXPRESSION OF ANGULAR FRINGE WIDTH............

Look at the figure below

here 'w' is the fringe difference and 'D' is the fringe-screen distance.

So, the angular fringe width would be

tanα = (w/2) / D

or

α = tan^-1 [w/2D]

thus, 

2α =  2.tan^-1 [w/2D]