The quickest path.

Light crossing into water takes the quickest route, not the shortest, and the quickest route obeys Snell's law. At the surface some light reflects and the rest passes, with none lost; for water, Brewster's angle is the 3-4-5 angle, and the bookkeeping becomes 7² + 24² = 25². Through N slits, waves add like arrows, and their sum's squared length is the brightness.