I am looking for the distance between the 2nd and 3rd DARK fringes on a screen. Monochromatic, coherent light is shone through 2 slits a distance d apart. Wavelength is L. Distance between grating and screen is R. The formula I am using is: [(R(5/2)L)/d - (R(3/2)L)/d] I am using 5/2 and 3/2 since these should be the number of wavelengths for destructive interference for the 3rd and 2nd fringes respectively. The formula doesn't seem to be working though. Where have I gone wrong????