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
Anticrepuscular rays Anticrepuscular rays Anticrepuscular rays, or antisolar rays, are atmospheric optical phenomena similar to crepuscular rays, but appear opposite of the Sun in the sky. Anticrepuscular rays are nearly parallel, but appear to converge toward the antisolar point due to linear perspective. Anticrepuscular rays are most frequently visible around sunrise or sunset. Appearing to radiate from the Sun, crepuscular rays usually look much brighter than anticrepuscular rays. This is because the atmospheric light scattering making the crepuscular rays visible occurs at low angles to the horizon (see Mie theory). Although anticrepuscular rays appear to converge toward the antisolar point, the convergence