Difference between Optical and Geometrical length?

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Optical length refers to the path light takes through a medium, influenced by the index of refraction, while geometrical length is the straight-line distance regardless of the medium. When the index of refraction is uniform and equal to one, optical length and geometrical length are the same. Optical path length is crucial in applications like interferometry, where it helps determine phase shifts between coherent light sources. Understanding the distinction is essential for solving problems in optical physics. The relationship between these lengths is foundational for various optical calculations and experiments.
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Difference between Optical and Geometrical length!?

I'm having this question since 2 times now in the theoretical part of the exam & still I couldn't find anything about it in the book neither in google.
Can anyone who's more experienced in Optical Physics give me the answer to this please !?
 
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Optical path length depends upon the index of refraction encountered; it is the same as geometrical length when the index of refraction is everywhere equal to 1.

See http://en.wikipedia.org/wiki/Optical_path_length

Optical path length is often used as the difference between two optical paths: then the difference in optical path length - this is used to calculate the phase shift between coherent light sources traveling the two paths, as in an interferometer.
 
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