In the winter of 2019 I stood inside the lantern room of the Cordouan lighthouse, face to face with a first-order Fresnel lens that had guided ships since 1823. The room was dark and cold. Then the keeper switched on the mechanism, and the lens caught a single bulb's worth of light and threw a coherent beam forty kilometers across black Atlantic water. The glass itself, thick with iron impurities, carried a faint aquamarine tint, as though the sea had been frozen into prisms.
The Architecture of Concentrated Light
Rather than wasting light in every direction, Fresnel's stepped-lens design used concentric annular prisms, each precisely angled to refract stray rays back toward the horizon. The central bullseye handled the direct beam; the surrounding rings caught and redirected what would otherwise scatter uselessly into the sky above or the water below. It was the first rigorous recycling of wasted energy, a principle that now underlies fiber optics and solar concentrators alike.