Optical Path Difference (OPD)
Optical path difference is the phase shift which happens between two previously coherent sources when passed through different mediums. For example a wave passed through glass will appear to travel a greater distance than an identical wave in air. This is because the source in the glass will have experienced a greater number of wavelengths due to the higher refractive index of the glass.
The OPD can be calculated from the following equation:
where d1 and d2 are the distances of the ray passing through medium 1 or 2, n1 is greater refractive index (e.g., glass) and n2 is the smaller refractive index (e.g., air).
Read more about this topic: Optical Path Length
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