PGRE | Michelson Interferometer Equation

[CanIExplore]

Homework Statement

I am having trouble understanding how I would apply the Michelson Interferometer equation to a problem on the PGRE. Here is a link to a diagram displaying the interferometer as well as the equation which is very similar to the way the topic is shown in my REA pgre book.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/michel.html

Homework Equations

2d=mλ

The Attempt at a Solution

I just don't understand what the variable d is referring to. In the REA book, it says that if the fixed mirror on the right is moved through some distance d, then the number of circular fringes m moves in. And using the aforementioned equation, you can solve for the wavelength of the light. Can anyone explain how this works?

 

[Delphi51]

d is the change in the distance L; it is how far the movable mirror moves.
2d is the change in the path the light takes going to the movable mirror and back.