concave lens

The passage of light through lenses. The concave lenses diverges a beam of light from a distant source. The convex and compound lenses focus light from a distant source to a point. The distance between the focus and the lens is called the focal length. The shorter the focus, the more powerful the lens.

A convex (or converging) lens causes light rays to refract inwards. A concave (or diverging lens) causes light rays to refract outwards. Convex lenses are used to correct long-sightedness and concave lenses to correct short-sightedness.

Lens that possesses at least one surface that curves inwards. It is a diverging lens, spreading out those light rays that have been refracted through it. A concave lens is thinner at its centre than at its edges, and is used to correct short-sightedness (myopia).

After light rays have passed through the lens, they appear to come from a point called the principal focus. The distance between the principle focus and the lens is the focal length. A more curved lens will have a smaller focal length and will be a more powerful lens. The image formed by a concave lens is virtual, upright, and smaller than the object, and it cannot be projected onto a screen. The lens formula is used to work out the position and nature of an image formed by a lens: 1/u + 1/v = 1/f, where u and v are the distances of the object and image from the lens, respectively, and f is the focal length of the lens.

Common forms of concave lens include the biconcave lens (with both surfaces curved inwards) and the planoconcave (with one flat surface and one concave). The whole lens may be further curved overall, making a convexo-concave or diverging meniscus lens, as in some lenses used in corrective eyewear.