I've been doing some working with a michelson interferometer, specifically finding the refractive index of a glass block. I am given this eqn: n = (2t-ML)(1-cos)/ (2t (1 - cos) - mL) Where n is the refractive index of the glass M is number of fringes passing I have left the thetas out becuase they mess up how it looks but u can assume cos = cos theta1. t is thickness of the glass L is Lambda the wavelength of the light. I am trying to prove this eqn. Here is what I have. I know that the optical path length is the distance the light travels in the glass * n. Where B is the distance travelled in the glass B = t cos theta2 Theta1 is angle of incidence, theta 2 is angle of refraction. From elsewhere L = 2d/M In this case B will be my d as this is the distance that the beam travel in excess of its normal path. Now I have tried just shoving everything in and hoping I can simplify it but to no avail. How should I be going about this?