- Any object which alters the behaviour of light passing through it by making it converge towards a focal point or diverge away from one. Lenses are usually made of glass or plastic. They work because light crossing the surface of the lens moves from a medium with one velocity of light to one where it has a different velocity, and alters direction as it does so. Accurate lenses for astronomical telescopes use ‘elements’ of different types of glass, mainly in an attempt to eliminate chromatic aberration. Lenses of unusual materials can be used to focus infrared and other types of radiation. Lenses are avoided for large telescopes, which tend to be reflectors, using mirrors, rather than refractors, with lenses, because they cannot be supported from behind and tend to sag under gravity, and because they are more costly to produce. With a mirror the image is formed by one surface, not several, so there is less expense and less scope for light to be absorbed, which happens each time light is reflected or refracted. For natural lenses in space.
- One or more objects, usually transparent and curved, which serve to refract and sometimes reflect light rays passing through. A lens causes parallel light beams to converge or diverge, is usually made of glass or plastic, and is utilized to form an image. Also called optical lens.
- A device utilized to focus an electron beam, analogous to the manner in which an optical lens focuses a light beam. electric and/or magnetic fields may be used for this purpose. Used, for instance, in CRTs, or in electron microscopes. Also called electron lens.
- A system of disks or other obstacles that refract sound waves similarly to the way an optical lens refracts light. Also called acoustic lens, or sound lens.
- A device which utilizes a magnetic field to focus a particle beam, analogous to the manner in which an optical lens focuses a light beam. Such a lens may consist, for instance, of an array of electromagnets. Also called magnetic lens.
- One or more objects, structures, or devices utilized to refract, diffract, or otherwise focus or shape any other form of electromagnetic radiation. An example is an antenna lens.
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