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/fˈoʊkəs/ /fˈoʊkɪs/ Listen

noun

Focus n. (pl. E. focuses, L. foci)

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.

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. 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.

3.

A central point; a point of concentration.

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 Roentgen 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.

verb

Focus v. t. (past & past part. focused or focussed; pres. part. focusing or focussing) To bring to a focus; to focalize; as, to focus a camera.

See images of 'Focus'

Etymology of 'Focus'

Collaborative International Dictionary of English 0.48

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"Focus" Quotes from Famous Books

... scheme many advantages are offered, which to the trader and mere man of the world are of considerable importance, by bringing all our charities to a focus. Setting aside the great saving that could and would be effected in the management by united efforts, a much larger sum might be given to the legitimate object of each charity, and a systematic and efficient check ...

— Suggestions to the Jews - for improvement in reference to their charities, education, - and general government • Unknown

... most. One other tree, yet fixed in the soil, presented an awful example of the might of the tornado. It was a chestnut of the largest size, the trunk near the base being seven or eight feet in circumference; it reclined at what seemed to have been the very focus of the whirlwind; its roots yet clung to earth; but, through the resistance thus offered, the tree had been literally twisted round and round, until it was split into laths, the trunk having the appearance of a great bundle of saplings peeled ...

— Impressions of America - During The Years 1833, 1834, and 1835. In Two Volumes, Volume II. • Tyrone Power

... application of photography to the study of the stars. The first photographic image of a star was obtained in 1850, by George P. Bond, with the assistance of Mr. J.A. Whipple, at the Harvard College Observatory. A daguerreotype plate was placed at the focus of the 15-inch equatorial, at that time one of the two largest refracting telescopes in the world. An image of [Greek: alpha] Lyrae was thus obtained, and for this Mr. Bond received a gold medal at the first international exhibition, that at the Crystal Palace, in ...

— The Future of Astronomy • Edward C. Pickering

... thoughts away from aborigines and heat, and tried to focus his mind elsewhere. He didn't understand psionic processes, he thought; but then, nobody did, really, as far as he knew. But ...

— Supermind • Gordon Randall Garrett

... have all nice, neat little clasped pray-books, and I have laid out 7s. 8d. in Watts's Hymns for Christmas presents for them. The eldest girl alone holds out; she has been at Boulogne, skirting upon the vast focus of Atheism, and imported bad principles in patois French. But the strongholds are crumbling. N. appears as yet to have but a confused notion of the Atonement. It makes him giddy, he says, to think much about it. But ...

— The Works of Charles and Mary Lamb (Vol. 6) - Letters 1821-1842 • Charles and Mary Lamb

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