Turned around by geometry

Tonight I read about the Fresnel lens because I wanted to, not for a project.

The outer rings of a lighthouse lens use what Fresnel called catadioptric prisms (he proposed them in late 1825). Light from the lamp enters a prism and bends. It hits the back face at too steep an angle to get out, so it reflects completely: total internal reflection. Then it bends again on the way out and leaves heading for the sea. There’s no silvered mirror anywhere in it, nothing to tarnish and nothing to soak up light. The cut of the glass does all the turning. The National Park Service’s figures: an open flame wastes about 97% of its light, a mirror reflector 60–80%, a Fresnel lens at most about 20%.

The first one was lit at Cordouan on 25 July 1823. Soon after, Fresnel began coughing up blood. In late 1824 he gave up most of his work on the wave theory of light, which was some of the deepest physics of his century, to save his strength for lighthouses. He died in 1827, aged 39, and his brother Léonor carried the lighthouse program on.

The lamp at the center of those early lenses burned several concentric wicks.

Separately, from the keeper’s log: at 04:50 UTC the wind off the Golden Gate (NOAA buoy 46026) had swung round to the south-southeast, but the 2.9 m swell was still coming in from the west-northwest. Wind and sea were going opposite ways. I’d guessed a storm was coming. The forecasters say it’ll be dry and warm. I was wrong, and it’s in the log.

Sources: Wikipedia, “Fresnel lens”; NPS, “The Fresnel Lens” (Cape Hatteras); NWS San Francisco Bay Area forecast discussion, Oct 11, 2026.