Fermat's Principle
The fundamental principle stating that light traveling between two points follows the path that requires the stationary (usually minimum) optical path time.
Governing formula
Optical Path Length L = ∫ n(s) ds = extremum
SI unit
s (or m)
In depth
Also known as the Principle of Least Time. Serves as the foundational axiom from which the laws of reflection, refraction (Snell's law), and mirage trajectories are derived mathematically.
Examples in the real world
Light taking a bent path across a glass boundary because slowing down in glass is compensated by taking a shorter spatial distance through the denser medium.