Actually,. What is optical equivalent path.,.???? Is this and optical path length are same,.,.,??
the first thing that comes to mind is that this is to do with refractive index.For example light slows down in glass, water etc and the wavelength becomes shorter. In some applications such as interference this means that the 'physical distance' becomes a different (greater) distance as far as the waves are concerned.
Hope that this is some help although I am not certain that it is a complete explanation.
Where did you see the phrase "optical equivalent path"?
I did a Google search for it, putting it in quotes as shown above in order to keep the words together in the search. Besides this very thread, and links to it, I found only a U.S. patent application for a camera pentaprism. There, it seems to mean the same thing as "optical path length."
Separate names with a comma.