Crystal ball Claude Alexander, was often billed as "Alexander the Crystal Seer". A crystal ball is essentially a bi-convex spherical lens with a uniform radius of curvature, although without its edges and center material truncated as in a conventional lens construction. Thus the principles of optics may be applied to analyze its imaging characteristics. As a lens, a transparent sphere of any material with refractive index greater than one bends rays of light to a focal point. An image is formed with significant coma, field curvature, and other optical aberrations inherent to spherical lenses. The refractive index of typical materials used for
Lens (optics) object off the optical axis of the lens is imaged, where rays pass through the lens at an angle to the axis θ. Rays that pass through the centre of a lens of focal length "f" are focused at a point with distance from the axis. Rays passing through the outer margins of the lens are focused at different points, either further from the axis (positive coma) or closer to the axis (negative coma). In general, a bundle of parallel rays passing through the lens at a fixed distance from the centre of the lens are focused to a ring-shaped