Can I have some help with Young's Slits as well, as I understand it, there are 2 equations. nlambda=dsintheta and X=Dlambda/d (some people use L instead of D). Which equation do you use for single slit, double slit and diffraction gratings. Currently I think single slit is the one involving X and double slit and diffraction grating both use the one involving sintheta. Some clarification would be appreciated