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
Refracting telescope principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye is able to collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6. The objective in a refracting telescope refracts or bends light. This refraction causes parallel light rays to converge at a focal point; while those not parallel converge upon a focal plane. The telescope converts a bundle of parallel rays to make an angle α, with the optical axis to a second parallel bundle