Optical Path Length and Refractive Index

Optical Path Length:

Optical path length is the distance traveled by light through a medium or a series of mediums. 

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In geometric optics, it is the product of the refractive index of the medium through which light is traveling and the physical distance the light travels in that medium. 

It is denoted by the symbol "d". The optical path length is used to determine the phase difference between two light rays and is important in calculating interference and diffraction effects in optics.

Refractive index: 

Refractive index is a measure of how much light slows down or bends as it passes through a substance. 

It is defined as the ratio of the speed of light in a vacuum to the speed of light in the substance. 

The refractive index of a material determines how much the path of light is bent when it enters or exits the material, which is why it is commonly used to describe how light interacts with different substances, such as glass or water.