If the two slits of double slit experiment were moved symmetrically apart with small relative velocity v and the distance between screen and mid-point of slits is fixed and equal to D. Consider a point P on the screen at a distance x from central maxima then (xltltltD):

Options

A. Rate of change of number of fringes between central maxima and point P changes with respect to time is `(xv)/(lambdaD)`
B. number of fringes contained between central maxima and point P increases with time
C. fringe width decreases as time passes
D. fringe width increases as time passes

Explanation

Let N be the number of fringes within the length x, then we have,
`betaN=xrArr (Dlambda)/dN=x rArr N=(xd)/(lambdaD)`
At any time t
`N=x/(lambdaD)(d+vt)`
`(dN)/(dt)=(xv)/(lambdaD)`