Using a diffraction grating as a example, demonstrate that the frequency difference of two maxima resolved according to Rayleigh's criterion is equal to the reciprocal of the difference of propagation times of the exterme interfering oscillations, i.e. `delta v = 1//delta t`.

Explanation

For the just resolved waves the frequecny difference
`delta v = (c delta lambda)/(lambda) = (c )/(lambdaR) = (c )/(lambdakN)`
`= (c )/(Nd sin theta) = (1)/(deltat)`
since `Nd sin theta` is the path difference between waves emitted by the extermities of the grating.