Principal focus

Principal focus
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.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • principal focus — noun : the focus for a beam of incident rays parallel to the axis * * * Optics. See focal point (def. 1). [1825 35] * * * principal focus (optics) The focus for rays parallel to the axis • • • Main Entry: ↑focus …   Useful english dictionary

  • principal focus — Optics. See focal point (def. 1). [1825 35] * * * …   Universalium

  • principal focus — /prɪnsəpəl ˈfoʊkəs/ (say prinsuhpuhl fohkuhs) noun → focal point (def. 1) …  

  • 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… …   The Collaborative International Dictionary of English

  • Focus tube — 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,… …   The Collaborative International Dictionary of English

  • 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 — {{Roman}}I.{{/Roman}} noun 1 special interest/attention ADJECTIVE ▪ central, main, major, primary, prime, principal, real ▪ important, speci …   Collocations dictionary

  • Focus (optics) — An image that is partially in focus, but mostly out of focus in varying degrees. In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge.[1] Although the focus is… …   Wikipedia

  • Principal Speaker — Principal Speakers were the public spokespersons of the Green Party of England and Wales but have since been replaced in the party by a national Leader and Deputy Leader. There were two Principal Speakers, one female and one male, who were… …   Wikipedia

  • Principal photography of The Lord of the Rings film trilogy — Principal photography for The Lord of the Rings film trilogy was conducted concurrently in New Zealand from October 11, 1999 through to December 22, 2000 for 274 days. Pick up shoots were conducted annually from 2001 to 2004. The trilogy was shot …   Wikipedia

Share the article and excerpts

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