A ball lens is an optical lens in the shape of a sphere. Formally, it is a bi-convex spherical lens with the same radius of curvature on both sides, and diameter equal to twice the radius of curvature. The same optical laws may be applied to analyze its imaging characteristics as for other lenses.
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As a lens, a transparent sphere of any material with refractive index (n) greater than air (n > 1.00) bends parallel rays of light to a focal point. For most glassy materials the focal point is only slightly beyond the surface of the ball, on the side opposite to where the rays entered. Ball lenses have extremely high optical aberration, including large amounts of coma and field curvature compared to conventional lenses.
Ball lenses or "lensballs" are used by photographers to take novel extreme wide-angle photos.