In $Y.D.S.E.$,the separation between the two slits $S_1$ and $S_2$ is $a$,and the screen is at a distance $x$ from the slits. The light source $S$ is moving with velocity $v$ parallel to the line joining $S_1$ and $S_2$ and is at a perpendicular distance $h$ from the plane of the slits. What is the frequency of change in brightness at the central point on the screen?

• A
  $\frac{h^2 v}{x^2 a}$
• B
  $\frac{v a}{\lambda h}$
• C
  $\frac{v^2}{x^2} \frac{a \lambda}{h}$
• D
  $\frac{v h}{2 \lambda a}$