Drakkith said:The optical path length is simply the distance the light travels times the refractive index. For example, light traveling through 10 cm of water has an optical path length of 10 x 1.333 or 13.3 cm. The geometric length is simply the physical distance the light travels.
The optical path length is important because it allows us to find out what the phase of the light will be at any point, whereas the geometrical length doesn't.
The optical length refers to the actual physical distance traveled by a light ray through a medium, while the geometrical length is the straight line distance between the start and end points of the light ray's path.
Optical length takes into account the refractive index of the medium, which affects the speed of light and therefore the distance traveled. Geometrical length only considers the straight line distance between two points.
The difference in optical and geometrical lengths determines the amount of refraction of light at the interface between two media, which can affect the path and direction of the light ray as it passes through.
An example would be a light ray traveling through a glass prism. The optical length would be longer than the geometrical length, as the light is traveling through a medium with a higher refractive index.
The optical length can be measured by taking into account the refractive index of the medium and the path traveled by the light ray. The geometrical length can be measured by directly measuring the straight line distance between the start and end points of the light ray's path.