1. The problem statement, all variables and given/known data I have attached the question to this thread as it requires looking at a diagram. 2. Relevant equations None 3. The attempt at a solution We are supposed to attempt the question and mark it but I have a few questions if people could kindly answer for me please. In the main 6 mark question I said in my answer that the path length difference from M and H would vary between 0 - 14mm (half a wavelength). If you look at the answer it says 0 - one full wavelength (28mm). I don't see how that is possible? If it is moved from plate M to 7mm away how could it vary anymore than half a wavelength. Also, the next 3 mark question requires using the path length difference of 14mm to get the answer. Unless it reflected of H again a second time but then if you go down that route it could reflect multiple times and have a path length difference greater than one wavelength. My second point of confusion is about other stuff the answer allows. It says allow full credit for (A^2+a^2)/(A^2-a^2). How is this calculation useful to the question? I get that when M and H are on top of each other the combined amplitude is A+a and when H is 7mm it would be A-a but how does finding a ratio of their squares help answer the question? Many thanks for any help given.