Optical path length

Optical path length is the vacuum distance light would travel in the time it takes to traverse a specified path through one or more materials. It weights each segment by the material’s refractive index.

Optical phase: The phase angle of an electromagnetic wave, which advances as the wave propagates. Optical path length determines phase accumulation at a given wavelength.
Michelson interferometer: An interferometer that splits light into two arms and recombines the returning beams. Its fringe shifts reveal changes in the arms’ optical path difference.
Speed of light: The speed at which electromagnetic radiation propagates in vacuum, exactly 299,792,458 metres per second. Optical path length converts travel time using this vacuum speed.
Geometric length: The ordinary spatial distance measured along a specified path. It ignores refractive index, so equal geometric lengths can have unequal optical path lengths.
Eikonal equation: A differential equation describing wavefront phase in the short-wavelength limit. Its phase field encodes optical path length through spatially varying media.
Thin-film interference: Interference between light reflected from different boundaries of a thin material layer. Reflections acquire phase differences set by the layer’s optical path length.
Travel time: The elapsed time taken to move between locations along a specified route. Optical path length is defined by the travel time light would cover at vacuum speed.
Optical path difference: The difference in optical path length between two specified routes taken by light. It compares paths; optical path length describes one path’s accumulated value.
Fermat's principle: The principle that light follows paths for which travel time is stationary under small variations. Stationary travel time is equivalent to stationary optical path length in a given medium.
Optical coherence tomography: An imaging method that uses low-coherence light to produce depth-resolved images of scattering samples. Interference measures optical delays that encode depth and refractive-index structure.

Linked from 23 pages