1. The problem statement, all variables and given/known data

Here is a worked problem:

I don't see why they've used "m-1/2" instead of "m+1/2"?

2. Relevant equations

According to my textbook the radius of m^{th} bright fringe is:

##x = ((m+\frac{1}{2})\lambda R)^{1/2}##

Where R is the radius of curvature of the convex lens.

3. The attempt at a solution

My textbook always uses "m+1/2" whereas my lecture notes always use "m-1/2". I'm confused because id I use the first one I get an entirely different answer.

I would appreciate it if any one could explain why we need to use the minus for this particular problem, and not the plus.

Thank you. I think if we start counting at ##m=0##, we should use ##m+\frac{1}{2}##. And if we count from ##m=1## then we should use ##m- \frac{1}{2}##.

BUT what if the problem asked for minima, instead of maxima?

I mean, the condition for minima is ##m \lambda##. Here do we count from m=0, or from m=1??