1. The problem statement, all variables and given/known data A beam of light is split into two coherent beams of intensity I which are in phase. The two beams are sent through different paths and superimposed at the same spot on a screen. A path difference is to be achieved by inserting a glass window of refractive index of 1.7 in the path of one of the beams while keeping the physical (actual) path lengths the same. How thick must the window be for an optical path difference of 4.5 wavelengths using light from a sodium vapour lamp (λ = 589.3 nm)? 2. Relevant equations OPD = t(n(s) - (n(m)) 3. The attempt at a solution n(s) = 1.7 n(m) = 1.0 (assuming it's in air.) OPD = 4.5 OPD/(n(s) - (n(m)) = t ...changing the equation around. 4.5/(1.7-1.0) = t t = 6.4 microns is this right?