Biconcave

A lens with two concave surfaces is biconcave (or just concave).

Between these articular surfaces is the articular disc (or meniscus), which is a biconcave, transversely oval disc composed of dense fibrous connective tissue.

If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens.

The broken pumice is clear and isotropic, and very small particles commonly have crescentic, sickle-shaped, or biconcave outlines, showing that they are produced by the shattering of a vesicular glass, sometimes described as ash-structure.