Optical path length is the length of the light that has to travel in the air to create the same phase difference.
It is the product of the geometric length of the light in the system and the refraction index of the medium through which it propagates.
For example: light traveling through of water will have optical length of
Geometric length is the physical distance of the light traveled.
Optical length tells us the phase of the light at any point, where geometric length does not.
Final Answer:
Optical path length is the length of the light that has to travel in the air to create the same phase difference.
Geometric length is the physical distance of the light traveled.
Optical length tells us the phase of the light at any point, where geometric length does not.
