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Optics Newton’s ring apparatus Problem

  1. Nov 8, 2013 #1
    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. jcsd
  3. Nov 8, 2013 #2


    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?
  4. Nov 8, 2013 #3
    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?? :confused:
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