Young experiment with white light

At point M the optical path difference   is :

The coherence length relative to the white source used (uniform source model) has a magnitude of :

The optical path difference is greater than the coherence length; consequently, the interference phenomenon can no longer be observed. Point M is located within the higher order white. By observing with a spectroscope, we can see that some wavelengths are missing.

The dark lines in the spectrum correspond to destructive interferences. Therefore, the orders of interference are verifying: .

They are included between the two extreme orders , kB and kR correspond to the two extremes of the spectrum. For the wavelength interval between and , the extreme values of the orders of interference are:

and

Since the orders of interference are not integers, there are only two dark lines in the spectrum that correspond to the orders , and therefore to the wavelengths:

and