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In optics, optical path length (OPL) or optical distance is the product of the geometric length of the path light follows through the system, and the index of refraction of the medium through which it propagates. A difference in optical path length between two paths is often called the optical path difference (OPD). Optical path length is important because it determines the phase of the light and governs interference and diffraction of light as it propagates. ==Optical path difference (OPD)== ''Optical path difference'' corresponds to 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 ''d''1 and ''d''2 are the distances of the ray passing through medium 1 or 2, ''n''1 is the greater refractive index (e.g., glass) and ''n''2 is the smaller refractive index (e.g., air). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「optical path length」の詳細全文を読む スポンサード リンク
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