Difference between Optical and Geometrical length?

[VeToMiNaToR]

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 !?

 

[UltrafastPED]

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.