# Optics Problem

1. Nov 8, 2013

### roam

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 mth 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.

2. Nov 8, 2013

### Staff: Mentor

Could there be a difference in how m is defined like is the very first ring m=0 or is it m=1

that is do they start counting at ZERO or at ONE?

3. Nov 8, 2013

### roam

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 $m- \frac{1}{2}$.

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