AP thin film Interference

Too many of you are still trotting out inappropriate formulae instead of looking at the physics. No reflection means destructive interference between waves reflected from the top of the coating and waves reflected from the bottom. This means that the round trip through the coating must take half a period and thus be equal to half a wavelength. Thus the governing equation is

2 x thickness = (1/2) wavelength in the coating or 2t = (1/2) (lambda vacuum/n coating)