The Lighthouse

optical bench — Fresnel lens

A thick converging lens bends parallel light to a focus — but most of the glass only adds bulk. Slice the lens into concentric zones, keep only the curved skin of each, and collapse them onto a flat sheet: the Fresnel lens. Same focus F, a fraction of the weight — the innovation that let a lighthouse throw a beam for miles.

Lens

Focal length f 2.20 m

Concentric rings N 10

Glass index n 1.490

View

Skin

The build: each ring's facet angle is solved by exact Snell's law so a parallel ray entering at that radius passes through F to machine precision — no spherical aberration, by construction. Watch the self-test prove it.