Lenses are classified by the curvature of the two optical surfaces. Also called a "diverging lens" because it diverges parallel rays outward. A convex lens pinches inward toward the centre of the lens. When this curve occurs on both sides of a lens it is called "biconcave" and when it occurs on a single side it is called "plano-concave".
Picture from www.shokabo.co.jp
Picture from www.shokabo.co.jp
