What is Optical Path Length?

In this video, I describe what optical path length is, in terms of how far light "thinks" it has traveled. I give an expression for the optical path length in terms of the refractive index, and a more general expression if the refractive index varies over space.
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This is part of my series on introductory electromagnetism, where we explore one of the fundamental forces of nature - how your phone charges and communicates with the rest of the world, why you should be afraid of the sun, and the fundamentals of electric and magnetic forces and fields, voltages,