With reference to the question in the image, the mark scheme for part (b)(i) is to take 130 mm / 4.…

With reference to the question in the image, the mark scheme for part (b)(i) is to take 130 mm / 4.5 = 29 mm. I do not understand do we need to divide by 4.5. It seems to me that the mark scheme is taking 130 mm = (n+1/2)*wavelength. While I could be wrong, I also doubt the validity of this equation because the path difference is not equal to 130 mm. Please help me!

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