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Augustin-Jean Fresnel developed wave descriptions of diffraction and polarization of an electromagnetic wave. Fresnel diffraction retains the curvature of the propagation wave phase, letting a wave account predict patterns at finite distances from an aperture. His polarization work helped establish the transverse character of light. In a modern example, orthogonal linear polarizations have no interference cross term in an ordinary total-intensity measurement; the field directions matter as well as the relative wave phase. He also connected optical theory with practical lighthouse illumination through the Fresnel lens.^[ref]

A physicist.^[ref]

A Fresnel lens divides the optical action of a thick lens into stepped sections, often concentric annular elements. Each section redirects light while omitting much of the material between the working surface and the corresponding continuous thick-lens shape. This permits a large collecting aperture with less bulk.

In a lighthouse, sections redirect rays from a small source into a concentrated beam. The arrangement can include refraction and reflection, and rotating the optical assembly produces characteristic flashes. The lens redistributes available light rather than creating optical power; source extent, losses and aberrations limit the beam. The preserved Anacapa lighthouse lens illustrates the large practical optical assemblies built on this principle.^[ref]

A lighthouse produces a visible navigational light from a fixed coastal or offshore structure. Its optical system concentrates light into selected directions rather than radiating equal useful intensity everywhere. A Fresnel lens can approximate the focusing action of a thick lens through stepped sections with less material. Rotating or switched beams give characteristic flash patterns that distinguish stations. Visibility depends on intensity, atmospheric scattering and the geometric horizon, so greater source power alone does not remove every range limitation.