Ok so the definition seems to be is the distance a wave would travel in air in the same time as it travelled in another medium.
But I don't understand why the formula is then opd=refractive index*geometrical path difference ??
Refractive index n=vc
Where c is the speed of light in a vacuum and v is the velocity through the medium.
Optical path length is defined as the product of the refractive index and the geometrical distance the light travels.
opl=nd
Optical path difference is a way of describing the phase shift between two paths emanating from the same coherent light source arriving at the same point after propagating different lengths and/or different mediums.
opd=n1d1−n2d2
If the same medium is propagated in both cases then the refractive index is common and can be taken outside brackets:
Where c is the speed of light in a vacuum and v is the velocity through the medium.
Optical path length is defined as the product of the refractive index and the geometrical distance the light travels.
opl=nd
Optical path difference is a way of describing the phase shift between two paths emanating from the same coherent light source arriving at the same point after propagating different lengths and/or different mediums.
opd=n1d1−n2d2
If the same medium is propagated in both cases then the refractive index is common and can be taken outside brackets: