planoconcave

A planoconcave lens has one flat surface and one surface that curves inward, like the inside of a sphere. The curved side is concave, meaning it diverges light rays rather than converging them. These lenses are negative lenses, producing virtual images that appear smaller and closer to the lens than the object itself.

The radius of curvature of the concave surface determines optical power, measured in diopters. A tighter curve (smaller radius) produces stronger divergence. The flat surface has infinite radius. In practice, planoconcave lenses are ground from optical glass, crown glass, or borosilicate, with the plane side often serving as a mounting surface because it sits flat against a cell or optical bench.

Planoconcave lenses appear in beam expanders, where they work alongside positive lenses to spread a collimated beam. They also correct spherical aberration in compound optical systems, reduce the effective focal length of an instrument without adding thickness, and serve as field flatteners in some imaging designs. Afocal systems, periscopes, and certain laser applications rely on them to achieve specific magnification or beam shape.

The plane surface makes assembly simpler than fully curved lenses. A planoconcave can be cemented directly to an optical bench or threaded cell without special spacers. However, the flat side is more vulnerable to dust and scratches during handling, since it has no protective curve. Anti-reflection coatings are often applied to both surfaces to minimize losses in multi-element systems.

Do not confuse planoconcave with planoconvex, which has one flat and one outward-curving surface and acts as a positive lens. The two are optically opposite and have different roles in optical design.