How can a lighthouse lens send a sweeping beam?

A lighthouse lamp sends light in many directions. Far away, a ship receives only a small share of that spreading light. A Fresnel lens redirects more of the lamp's light toward a useful beam, making a distant observer's direction brighter without making the lamp produce extra light.

A thick lens would need a great deal of glass. A Fresnel design divides the needed shapes into thinner stepped sections. In a lighthouse optic, a central lens panel and surrounding glass sections collect rays leaving at different angles. Refraction bends their paths; some lighthouse sections also use internal reflection. The shape of each section determines where its light goes.

The optic can revolve around a steadily shining lamp. A ship sees a bright flash when a beam sweeps across it, then much less light as the beam points elsewhere. The observer sees a repeating signal even though the lamp has never switched off.

The number of beam panels and the speed of rotation help set the pattern of flashes. This is a useful distinction between making light and directing it: the lamp supplies the light, the glass gathers it into selected directions, and rotation turns those directions into a pattern over time. The drawn rays are idealized and do not establish a particular lighthouse's visibility distance.