1. The problem statement, all variables and given/known data I'm having trouble understanding how you are supposed to decide which nodal and antinodal line numbers are supposed to be attached to the corresponding line. 2. Relevant equations https://roncalliphysics.wikispaces.com/Year+13+Double+Slit+Interference I've read this and other links, and also the information provided to complete the work. In the school book it says anti nodal lines should be m and only m should be in the center with a value of 0. I'm talking about these numbers here and how they apply to path difference and wavelength equations. 3. The attempt at a solution From what I read at the link it sounds like P should be n=2... From what I read in my school book it sounds like for P n=3... It also gives integer values to the anti nodal lines instead of decimals. The answer is really going to determine the grade so I need to know which one is right?