Editing Optical path length (section)

== Optical path difference ==

The OPD corresponds to the [[phase shift]] undergone by the light emitted from two previously [[Coherence (physics)|coherent]] sources when passed through mediums of different [[Refractive index|refractive indices]]. For example, a wave passing through air appears to travel a shorter distance than an identical wave traveling the same distance in glass. This is because a larger number of wavelengths fit in the same distance due to the higher [[refractive index]] of the [[glass]].

The OPD can be calculated from the following equation:

:<math>\mathrm{OPD}= d_1 n_1 - d_2 n_2</math>
where ''d''<sub>1</sub> and ''d''<sub>2</sub> are the distances of the [[Optical ray|ray]] passing through medium 1 or 2, ''n''<sub>1</sub> is the greater refractive index (e.g., glass) and ''n''<sub>2</sub> is the smaller refractive index (e.g., air).