1. The problem statement, all variables and given/known data 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 2. Relevant equations 2d=mλ 3. 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?