The rippling bright net of light you see on the bottom of a sunlit pool is a caustic — a pattern formed where refracted light rays are focused or spread out by a curved surface. Every point on the moving water surface tilts by a different amount, and each tilt bends the sunlight passing through it by a different angle, following Snell's law: n₁sinθ₁ = n₂sinθ₂. Where many nearby rays get bent toward the same spot on the floor, light concentrates into a bright streak; where rays are bent apart, a dim shadow-like gap appears.
Caustics also form the shimmering "sun ring" seen from below the water surface when looking up, and the same optics explain the bright rings inside a coffee cup and the crescent-shaped patches of light under leafy trees during a solar eclipse.
An interactive 3D pool where a rippling water surface bends sunlight according to Snell's law, tracing each refracted ray down to the floor to build the shifting bright-and-dark caustic net swimmers see underwater.
Local surface tilt determines how much each point on the water bends incoming light. Rays refracted toward a common spot pile up into bright caustic lines; rays bent apart leave dim gaps — all governed by n₁sinθ₁ = n₂sinθ₂.
Raise wave amplitude to sharpen the caustic net, tilt the sun angle to shift the pattern, and change pool depth to see the caustic focus tighten or blur. Toggle ray lines to see individual refracted paths.
The same caustic optics that light up a pool floor also form rainbows, the bright ring inside a coffee cup, and the crescent-shaped sunlight patches under trees during a partial solar eclipse.