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
Focal cloud antennas might benefit from minor deformation, producing a footprint encompassing satellite tidal motion. Focal cloud A focal cloud is the collection of focal points of an "imperfect" lens or parabolic reflector whether optical, electrostatic or electromagnetic. This includes parabolic antennas and lens-type reflective antennas of all kinds. The effect is analogous to the circle of confusion in photography. In a perfect lens or parabolic reflector, rays parallel to the device's axis striking the lens or reflector all pass through a single point, the focal point. In an imperfectly constructed lens or reflector, rays passing through different parts of the element