one is this another is additional path it is traversing through the thin film and then if you draw a normal; this path will be equal. path difference will be the delta will be AB, AB plus BC AB plus BC into mu it is the if this is the refractive index of this thin film into mu minus AD, it is near outside if it is a minus AD, right.
இவற்றிற்கு ஒரு நார்மல் வரைந்தோம் என்றால் அதனுடைய பாதை பாதை வேறுபாடு = ẟ = μ