biconcave

A biconcave lens is a piece of optical glass or plastic ground so that both its surfaces curve inward, away from the center. This geometry makes it a diverging lens, spreading parallel light rays outward as though they came from a single point behind the lens. The opposite of a biconvex lens, which converges light.

The strength of a biconcave lens depends on the radius of curvature of each surface and the refractive index of the material, usually expressed in diopters (reciprocal meters). A lens with tighter curves diverges light more sharply. Most biconcave lenses in practice are made from borosilicate glass or optical-grade plastic, with spherical surfaces ground to tolerances of a few micrometers in precision work.

Where biconcave lenses appear

Biconcave lenses correct myopia (nearsightedness) in eyeglasses and are a standard element in camera viewfinders, rifle scopes, and microscope eyepieces. They also appear as field lenses in projection systems and as elements in apochromatic objectives to reduce chromatic aberration. In laser systems, a biconcave lens can expand a beam or match its divergence to a target.

The main practical problem with biconcave lenses is edge thickness. Because material is thinnest at the center and thickest at the rim, a biconcave lens used at large apertures can introduce edge distortion or vignetting. They are also more prone to breakage than plano-convex designs because stress concentrates at the thin center. Tinting or coating the lens to reduce reflections is common; uncoated glass surfaces reflect about 4 percent of light at each air-glass boundary.

The term comes from the Latin bi (two) and concavus (hollowed). Optical workers often call them minus lenses or negative lenses because they have negative focal length, a convention that reflects their effect on light path rather than their shape.