I'm during an experiment involving the Michelson- Interferometer, now i need to find the uncertainty for lambda using the equation. 2 (path length difference) = N (lambda) Now I no the above equation can be changed to 2(path length difference)/ N = Lambda; and from their the corresponding equation to find the uncertainty in lambda is (uncertainty in lambda)/ lambda = ( (uncertainty in path length difference)/path length difference)^2 + ( (uncertainty in fringe count)/fringe count )^2 )^ (1/2) Path length difference is a free variable meaning It's not a specific value and the same can be said of lambda. So what I decided to do is find the corresponding lambda for each path length difference and plug A conjugate pair into the equation. The problem is that when I do that I get different values for the uncertainty. What am I doing wrong.