Nodal and anti-nodal lines in wave interference patterns

[julianwitkowski]

Random question about waves like this as they pertain to |a-b| = λ(n - ½).

Is the middle nodal line = 1, since it is 2 over, would P_n = 3 in this image.

If that is not right, the middle nodal line must = 0, and therefore P_n = 2.

I don't think this is true because I remember reading the middle nodal line is never 0, only an anti-nodal line, but I might be wrong.

 

[jedishrfu]

This may help

https://roncalliphysics.wikispaces.com/Year+13+Double+Slit+Interference