Fresnel Equations
Definition
How much light reflects versus transmits at a boundary — why windows show your reflection AND the view beyond.
Technical definition
Amplitude coefficients r_s, r_p, t_s, t_p for s- and p-polarizations from matching E and B fields at the interface; reflectance R = |r|².
Example
Polarized sunglasses exploit the Fresnel equations — at Brewster's angle (~53° on water) reflected glare is fully polarized and gets blocked.
Common confusion
The Fresnel equations don't contradict Snell's law — Snell gives the DIRECTION, Fresnel gives the AMOUNT. You need both for the full picture.