1. The problem statement, all variables and given/known data A source S of monochromatic light and a detector D are both located in air a distance of h above a horizontal plane sheet of glass, and are separated by a horizontal distance of x. Waves reaching D directly from S interfere with waves that reflect off the glass. The distance x is small enough compared to h so that the reflection is at close to normal incidence. 2. Relevant equations 2t = (m+1/2) lambda 3. The attempt at a solution I know how to solve this problem if I understand one concept. That is, what does the "t" value exactly refer to? I understand that when using thin lens, it refers to the thickness of a lens. But in the scenario above, what does the t value exactly refer to when there is no physical presence of a lens? Is it the distance b/w the two sources?