This is for my Optics class. I am having a lot of trouble understanding this concept and my professor will not answer any of my emails. I do not know where else to turn. The concept deals with an air gap between microscope slides or something. I believe it has something to do with fringes of equal thickness but I do not understand what this term means. Below is a problem that I have no idea how to solve because it uses this concept. How does the air gap play a role? I REALLY need help! 1. The problem statement, all variables and given/known data A beam of white light from 400-700 nm is incident at 45 degrees on two parallel glass plates separated by a 0.001cm air gap.The reflected light is admitted into a prism spectroscope. How many dark lines are seen on the entire spectrum. 2. Relevant equations I believe I am suppose to use this equation somehow. [tex]2n_f tcos\theta_r=(m+1/2)\lambda[/tex] Where [itex]n_f[/itex] is the refractive index of the film. "t" is the thickness of the film and [itex]\theta_r[/itex] is the angle of refraction. I set it equal to (m+1/2)λ so it would be constructive interference. 3. The attempt at a solution I do not know where to start as I don't know how to deal with the air gap! Can anyone help? :(???