Focus Fo"cus (f[=o]"k[u^]s), n.; pl. E. {Focuses} (f[=o]"k[u^]s*[e^]z), L. {Foci} (f[=o]"s[imac]). [L. focus hearth, fireplace; perh. akin to E. bake. Cf. {Curfew}, {Fuel}, {Fusil} the firearm.] 1. (Opt.) A point in which the rays of light meet, after being reflected or refracted, and at which the image is formed; as, the focus of a lens or mirror. [1913 Webster]

2. (Geom.) A point so related to a conic section and certain straight line called the directrix that the ratio of the distance between any point of the curve and the focus to the distance of the same point from the directrix is constant. [1913 Webster]

Note: Thus, in the ellipse FGHKLM, A is the focus and CD the directrix, when the ratios FA:FE, GA:GD, MA:MC, etc., are all equal. So in the hyperbola, A is the focus and CD the directrix when the ratio HA:HK is constant for all points of the curve; and in the parabola, A is the focus and CD the directrix when the ratio BA:BC is constant. In the ellipse this ratio is less than unity, in the parabola equal to unity, and in the hyperbola greater than unity. The ellipse and hyperbola have each two foci, and two corresponding directrixes, and the parabola has one focus and one directrix. In the ellipse the sum of the two lines from any point of the curve to the two foci is constant; that is: AG + GB = AH + HB; and in the hyperbola the difference of the corresponding lines is constant. The diameter which passes through the foci of the ellipse is the major axis. The diameter which being produced passes through the foci of the hyperbola is the transverse axis. The middle point of the major or the transverse axis is the center of the curve. Certain other curves, as the lemniscate and the Cartesian ovals, have points called foci, possessing properties similar to those of the foci of conic sections. In an ellipse, rays of light coming from one focus, and reflected from the curve, proceed in lines directed toward the other; in an hyperbola, in lines directed from the other; in a parabola, rays from the focus, after reflection at the curve, proceed in lines parallel to the axis. Thus rays from A in the ellipse are reflected to B; rays from A in the hyperbola are reflected toward L and M away from B. [1913 Webster]

3. A central point; a point of concentration. [1913 Webster]

{Aplanatic focus}. (Opt.) See under {Aplanatic}.

{Conjugate focus} (Opt.), the focus for rays which have a sensible divergence, as from a near object; -- so called because the positions of the object and its image are interchangeable.

{Focus tube} (Phys.), a vacuum tube for R[oe]ntgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect.

{Principal focus}, or {Solar focus} (Opt.), the focus for parallel rays. [1913 Webster]

The Collaborative International Dictionary of English. 2000.


Look at other dictionaries:

  • FOCUS — Beschreibung Nachrichtenmagazin Verlag Hubert Burda Media …   Deutsch Wikipedia

  • Focus — Saltar a navegación, búsqueda Para otros usos de este término véase Focus (desambiguación). Focus Información personal Origen Holanda Información artística …   Wikipedia Español

  • Focus — Beschreibung Nachrichtenmagazin Sprache …   Deutsch Wikipedia

  • Focus — may refer to:In science, mathematics or computing: *Focus (optics), a point toward which light rays are made to converge **Autofocus, a feature of some optical systems that obtains and maintains correct focus on a target **Focal length, a measure …   Wikipedia

  • FOCUS — is a software product of Information Builders Inc. Originally developed for data handling and analysis on the IBM mainframe, as newer systems were developed and smaller computers became more powerful, the available platforms for FOCUS were… …   Wikipedia

  • focus — fo‧cus [ˈfəʊkəs ǁ ˈfoʊ ] noun [uncountable] MARKETING when a company tries to serve particular groups of customers in a market with particular needs, rather than trying to serve the whole market focuser noun [countable] : • The focuser seeks to… …   Financial and business terms

  • focus — [fō′kəs] n. pl. focuses or foci [fō′sī΄] [ModL, adopted in math. senses by KEPLER Johannes (1604) < L, fireplace, hearth < ? IE base * bhok , to flame, burn > ? Arm boc̣, flame] 1. the point where rays of light, heat, etc. or waves of… …   English World dictionary

  • FOCUS — микропроцессор производства компании 1982 году. Стал первым коммерческим, однокристальным, полностью 32 битным ЦПУ выпущенным на рынок. В то время все 32 битные микропроцессоры конкурентов (DEC, микросхем, составлявших модульную конструкцию ЦПУ.… …   Википедия

  • FOCUS — Servio a fovendo dictus est quidquid fovet ignem, sive ara sit, sive quidquid aliud, in quo ignis fovetur. Peculiariter vox sumitur pro ara Diis domesticis, quos Penates Laresque olim dixêrunt, sacra. Unde Plaut. Aulul. Act. 2. Sc. 8. v. 16. Haec …   Hofmann J. Lexicon universale

  • focus — FÓCUS s.n. 1. Bacterii, localizate cel mai adesea în gură şi faringe, care răspândesc în sânge focare de infecţie. 2. Focar, la lentile şi oglinzi. [cf. germ. Fokus, fr. focus]. Trimis de LauraGellner, 07.04.2005. Sursa: DN  FÓCUS s. n. 1.… …   Dicționar Român

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”