In standard YDSE slits were moved apart symmetrically with relative velocity v, calculate the rate at which fringes pass a point at a distance x from the centre of the fringe system formed on a screen at 'y' distance away from the double slits if wavelength of light is `lambda` , and distance between the slits is 'b' . Assuming `y gtgt b & b gtgt lambda`

Options

A. `(2xv)/(lambda y)`
B. `(xv)/(2lambda y)`
C. `(xv)/(lambda y)`
D. Zero

Answer

C

Explanation

No.of fringes passing through point 'P' = no.of maxima passing through 'p'
path difference at 'p'
`Delta x = (xd)/(y)`
For maxima `Delta x = n lambda`
`n lambda = (xd)/(y)`
`(dn)/(dt) = (xv)/(y lambda)`