High Quality Content by WIKIPEDIA articles! In geometry, a spherical cap is a portion of a sphere cut off by a plane. If the plane passes through the center of the sphere, so that the height of the cap is equal to the radius of the sphere, the spherical cap is called a hemisphere. If the radius of the sphere is called r, the radius of the base of the cap called a, and the height of the cap called h, the volume of the spherical cap is then V = h(3a2 + h2) / 6 and the curved surface area of the spherical cap is A = 2 rh.