Young's double slit produces fringe patterns that are equally spaced having fringe separation, x. The phase difference of the light arriving at n=0 or n=1 or n=2 or n=3 etc are said to have zero phase difference or phase angle = 0. The path difference of the light arriving at n = 0 is zero, for n=1 is one wavelength , and n=2 is two wavelength, n=3 is three wavelength etc.
Apart from these very distinct cases , what is the phase difference for a location mid pt between n=2 and n=3 bright fringes. If n=2 and n=3 has a phase angle =0 or 360', then mid point will have to be 180'. I state degree because I cant find pi on my keyboard. The path difference will have to be 2.5 wavelength.
For another less obvious case where it is neither bright nor dark, and located halfway between a bright and dark fringe, then the phase angle would be halved again to 90' and the path difference will have to be 2.25 wavelength or 2.75 wavelength since we are looking between n=2 and n=3.
