Lens (optics) concave surfaces is "biconcave" (or just "concave"). If one of the surfaces is flat, the lens is "plano-convex" or "plano-concave" depending on the curvature of the other surface. A lens with one convex and one concave side is "convex-concave" or "meniscus". It is this type of lens that is most commonly used in corrective lenses. If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot (a "focus") behind the lens. In this case, the lens is called a "positive" or "converging" lens. The distance from the lens to the spot
Crepuscular rays Crepuscular rays Crepuscular rays (more commonly known as sunbeams, sun rays, splintered light, or god rays), in atmospheric optics, are rays of sunlight that appear to radiate from the point in the sky where the Sun is located. Shining through openings in clouds (particularly stratocumulus) or between other objects, these columns of sunlit particulates are separated by darker cloud-shadowed volumes. Despite seeming to converge toward the light source, the rays are in fact near-parallel shafts of sunlight. Their apparent convergence is a perspective effect. This phenomenon mimics the way parallel railway lines or long hallways, for example, seem to seek