Ball Lens

The "curvature of the sphere" of a spherical lens is constant, that is, the front and back surfaces of the lens are each part of a spherical surface. Each surface can be convex (raised outward from the lens), concave (recessed into the lens), or "planar" (flat). The line connecting the spherical center points of the front and rear surfaces of the lens is called the optical axis of the lens. In almost all cases, the optical axis of the lens will pass through the physical center of the lens.