Curving inwards. A concave mirror is one in which the reflecting surface is formed from the interior surface of a sphere or paraboloid. A concave lens has at least one face formed from the interior surface of a sphere. A biconcave lens has both faces concave and is therefore thinnest at its centre. The plano-concave lens has one plane face and one concave face. The concavo-convex lens (also called a meniscus) has one concave face and one convex face. See lens.