Hello and thank you! I am having trouble understanding Mr. Thomas Young's equations for the double-slit apparatus with the bands etc. Specifically, for constructive interference: Length = PS1-PS2 = n(lamda) What I understand is that this n value is the phase shift in their wavelength. (i.e. one whole wavelength, a half a wavelength, etc.) But for destructive interference, the formula is: Length = PS1-PS2 = (n-0.5)(lamda) Why is 0.5 being subtracted? I do not understand conceptually.