focal pointn.

Brit.

/ˈfəʊkl ˌpɔɪnt/, U.S.

/ˈfoʊkəl ˌpɔɪnt/

Origin: Formed within English, by compounding. Etymons: focal adj., point n.¹

Etymology: < focal adj. + point n.¹ Compare earlier focus n. 5 Compare also slightly earlier focal length n. at focal adj. Compounds and focal distance n.

1. Optics.

a. The point (or either of the points: see note) on the focal axis of a curved mirror, lens, or lens system at which rays initially parallel to the focal axis converge (or from which they appear to diverge) after reflection or refraction; = focus n. 5a, principal focus n. at principal adj., n., and adv. Compounds.A mirror with spherical or parabolic curvature has one focal point; a lens has two, one on each side.For a lens (or combination of lenses) in which chromatic aberration has not been corrected, electromagnetic radiation of different wavelengths will have different focal points.

ΘΚΠ

the world > physical sensation > sight and vision > optical instruments > lens > [noun] > focal length or point

focus1664

focus1666

focal distance1684

focal point1692

burning-point1698

short focusa1830

visual focus1837

the world > matter > physics > electromagnetic radiation > heat > [noun] > emission or diffusion > energy produced by > directed flow > meeting point of

focus1664

focal point1692

1692 W. Molyneux Dioptrica Nova i. 78 We know the Focal Point is some little nigher the Glass in the Parallel Rays that fall widest from the Axis, than in those that fall near it... But the difference is so small..it may be taken as a simple Point.

1742 Philos. Trans. 1739–40 (Royal Soc.) 41 339 This Error may be rectified by one or more Glasses, placed in the Axis, at a Distance farther from the Vertex than the Focal Point.

1763 B. Martin Young Gentleman & Lady's Philos. II. v. xvii. 316 There is one Case yet remaining..: And that is, when the Object is placed between the focal Point and the Glass itself.

1804 J. Webster Elem. Nat. Philos. 236 When the radient, or object, is at a considerable distance from the lens, the rays issuing from it will fall upon the glass and converge in the focal point.

1895 R. S. Heath Geom. Optics (ed. 2) 83 It is clear that F is the first focal point of the combined system.

1926 Michigan Technic May 29/2 A two-filament lamp is mounted in a standard reflector in such a way that one filament is exactly at the focal point.

1982 M. Millodot in H. B. Barlow & J. D. Mollon Senses iv. 72 The optical correction of a myope consists of a negative lens since this lens has a virtual focal point in front of the eye.

2005 Flight Internat. 10 May 36/1 A localised plasma..forms an RF [= radio-frequency] mirror, which deflects a radar beam generated at the mirror's focal point.

b. Any point at which rays converge (or from which rays appear to diverge) in reflection or refraction; = focus n. 5b.In quot. 1713 with reference to hypothetical ‘sonorous rays’ of sound behaving like rays of light. Cf. focus n. 3.

ΘΚΠ

the world > matter > physics > electromagnetic radiation > light > emission of light, radiation > [noun] > point from which rays proceed

radiant point1665

focus1704

focal point1713

radiant1728

radiator1811

point source1894

the world > matter > physics > electromagnetic radiation > light > emission of light, radiation > [noun] > meeting point

focus1664

visual point1679

focal point1713

chemical focus1841

point focus1908

1713 W. Derham Physico-theol. iv. iii. 126 Whether the Convexity or Concavity of the Drum collects those Rays into a focal Point, or scatter them.

a1774 O. Goldsmith Surv. Exper. Philos. (1776) II. iii. vi. 303 By the time it [sc. a visual ray] has passed thence it is nearly collected into a focal point, but still converging yet more as it proceeds through the vitreous humour.

1801 T. Belsham Elem. Philos. Mind iv. 79 When the radiant points are contiguous to each other, the corresponding focal points will also be contiguous.

1865 N. Arnott Elements Physics (ed. 6) iv. ii. 519 The lens..concentrates all [rays] into focal points, which form a general image of great vividness and beauty.

1898 Jrnl. Royal Microsc. Soc. 402 When a small oblique pencil is refracted by a lens, the rays are not brought to a focal point, but to a line.

1955 Sci. Amer. Mar. 38/1 A parabolic ‘dish’, either solid or made of a wire screen, reflects incoming radio waves to a focal point.

1996 New Scientist 17 Feb. 28/2 At the focal point [of the microwave or laser beam], the concentration of energy is high enough to rip electrons from molecules in the air to form a plasma.

2012 New Scientist 31 Mar. 20/3 The [photon] sieve is an ultra-thin plastic disc perforated by millions of microscopic holes, each of which bends light at different angles to create a focal point.

2. figurative and in extended use. A point at which a number of different elements converge; (now usually) a centre of interest, attention, or activity. Cf. focus n. 6a, 7b.

ΘΚΠ

the mind > attention and judgement > attention > earnest attention, concentration > [noun] > focus of attention

centre1582

focus1751

focal point1755

nodal point1862

centre stage1887

1755 E. Young Centaur v. 283 Human thought..whose scattered rays must be collected, as it were, to a focal point, in order duly to warm our devotion.

1855 T. B. Macaulay Hist. Eng. IV. 175 Titus, in order to be near the focal point of political intrigue and faction, had taken a house within the precinct of Whitehall.

1937 Amer. Home Apr. 115/2 Their novelty makes them a decided focal point and..they [sc. miniature buildings] are much more charming when displayed alone.

1974 J. Blish Star Trek 10 45 Somehow you are the crux..the focal point of all this.

2013 Fast Company June 41/1 We're kitting out a new office space with a mix of new furniture, salvaged pieces, and some custom-made focal points.

3. Geometry. Any of various points whose mathematical relationship to a given geometric figure is regarded as analogous to that between a conic section and its focus or foci (now rare); (in later use usually) a focus of a conic section.

ΚΠ

1844 Proc. Royal Irish Acad. 1840–4 2 469 It is remarkable that the vertex of the cone affords the only instance of a focal point which is at once modular and umbilicar.

1868 Q. Jrnl. Pure & Appl. Math. 9 320 The line joining it to a point exterior to the conicoid is the axis of the cone of which the focal point is the vertex.

1919 Ann. Math. 20 233 The four circumscribed circles are concurrent at a point. This will be called the focal point of the quadrilateral.

1964 R. K. Luneburg Math. Theory Optics 170 It follows that the point x = y = 0 is a common focal point of all the conics.

2003 H. Schaub & J. L. Junkins Analyt. Mech. Space Syst. ix. 383 The sum of the two radial distances from any point on the ellipse to each focal point is constant.

This entry has been updated (OED Third Edition, September 2016; most recently modified version published online March 2022).

单词	focal point
释义	focal pointn. Brit. /ˈfəʊkl ˌpɔɪnt/, U.S. /ˈfoʊkəl ˌpɔɪnt/ Origin: Formed within English, by compounding. Etymons: focal adj., point n.¹ Etymology: < focal adj. + point n.¹ Compare earlier focus n. 5 Compare also slightly earlier focal length n. at focal adj. Compounds and focal distance n. 1. Optics. a. The point (or either of the points: see note) on the focal axis of a curved mirror, lens, or lens system at which rays initially parallel to the focal axis converge (or from which they appear to diverge) after reflection or refraction; = focus n. 5a, principal focus n. at principal adj., n., and adv. Compounds.A mirror with spherical or parabolic curvature has one focal point; a lens has two, one on each side.For a lens (or combination of lenses) in which chromatic aberration has not been corrected, electromagnetic radiation of different wavelengths will have different focal points. ΘΚΠ the world > physical sensation > sight and vision > optical instruments > lens > [noun] > focal length or point focus1664 focus1666 focal distance1684 focal point1692 burning-point1698 short focusa1830 visual focus1837 the world > matter > physics > electromagnetic radiation > heat > [noun] > emission or diffusion > energy produced by > directed flow > meeting point of focus1664 focal point1692 1692 W. Molyneux Dioptrica Nova i. 78 We know the Focal Point is some little nigher the Glass in the Parallel Rays that fall widest from the Axis, than in those that fall near it... But the difference is so small..it may be taken as a simple Point. 1742 Philos. Trans. 1739–40 (Royal Soc.) 41 339 This Error may be rectified by one or more Glasses, placed in the Axis, at a Distance farther from the Vertex than the Focal Point. 1763 B. Martin Young Gentleman & Lady's Philos. II. v. xvii. 316 There is one Case yet remaining..: And that is, when the Object is placed between the focal Point and the Glass itself. 1804 J. Webster Elem. Nat. Philos. 236 When the radient, or object, is at a considerable distance from the lens, the rays issuing from it will fall upon the glass and converge in the focal point. 1895 R. S. Heath Geom. Optics (ed. 2) 83 It is clear that F is the first focal point of the combined system. 1926 Michigan Technic May 29/2 A two-filament lamp is mounted in a standard reflector in such a way that one filament is exactly at the focal point. 1982 M. Millodot in H. B. Barlow & J. D. Mollon Senses iv. 72 The optical correction of a myope consists of a negative lens since this lens has a virtual focal point in front of the eye. 2005 Flight Internat. 10 May 36/1 A localised plasma..forms an RF [= radio-frequency] mirror, which deflects a radar beam generated at the mirror's focal point. b. Any point at which rays converge (or from which rays appear to diverge) in reflection or refraction; = focus n. 5b.In quot. 1713 with reference to hypothetical ‘sonorous rays’ of sound behaving like rays of light. Cf. focus n. 3. ΘΚΠ the world > matter > physics > electromagnetic radiation > light > emission of light, radiation > [noun] > point from which rays proceed radiant point1665 focus1704 focal point1713 radiant1728 radiator1811 point source1894 the world > matter > physics > electromagnetic radiation > light > emission of light, radiation > [noun] > meeting point focus1664 visual point1679 focal point1713 chemical focus1841 point focus1908 1713 W. Derham Physico-theol. iv. iii. 126 Whether the Convexity or Concavity of the Drum collects those Rays into a focal Point, or scatter them. a1774 O. Goldsmith Surv. Exper. Philos. (1776) II. iii. vi. 303 By the time it [sc. a visual ray] has passed thence it is nearly collected into a focal point, but still converging yet more as it proceeds through the vitreous humour. 1801 T. Belsham Elem. Philos. Mind iv. 79 When the radiant points are contiguous to each other, the corresponding focal points will also be contiguous. 1865 N. Arnott Elements Physics (ed. 6) iv. ii. 519 The lens..concentrates all [rays] into focal points, which form a general image of great vividness and beauty. 1898 Jrnl. Royal Microsc. Soc. 402 When a small oblique pencil is refracted by a lens, the rays are not brought to a focal point, but to a line. 1955 Sci. Amer. Mar. 38/1 A parabolic ‘dish’, either solid or made of a wire screen, reflects incoming radio waves to a focal point. 1996 New Scientist 17 Feb. 28/2 At the focal point [of the microwave or laser beam], the concentration of energy is high enough to rip electrons from molecules in the air to form a plasma. 2012 New Scientist 31 Mar. 20/3 The [photon] sieve is an ultra-thin plastic disc perforated by millions of microscopic holes, each of which bends light at different angles to create a focal point. 2. figurative and in extended use. A point at which a number of different elements converge; (now usually) a centre of interest, attention, or activity. Cf. focus n. 6a, 7b. ΘΚΠ the mind > attention and judgement > attention > earnest attention, concentration > [noun] > focus of attention centre1582 focus1751 focal point1755 nodal point1862 centre stage1887 1755 E. Young Centaur v. 283 Human thought..whose scattered rays must be collected, as it were, to a focal point, in order duly to warm our devotion. 1855 T. B. Macaulay Hist. Eng. IV. 175 Titus, in order to be near the focal point of political intrigue and faction, had taken a house within the precinct of Whitehall. 1937 Amer. Home Apr. 115/2 Their novelty makes them a decided focal point and..they [sc. miniature buildings] are much more charming when displayed alone. 1974 J. Blish Star Trek 10 45 Somehow you are the crux..the focal point of all this. 2013 Fast Company June 41/1 We're kitting out a new office space with a mix of new furniture, salvaged pieces, and some custom-made focal points. 3. Geometry. Any of various points whose mathematical relationship to a given geometric figure is regarded as analogous to that between a conic section and its focus or foci (now rare); (in later use usually) a focus of a conic section. ΚΠ 1844 Proc. Royal Irish Acad. 1840–4 2 469 It is remarkable that the vertex of the cone affords the only instance of a focal point which is at once modular and umbilicar. 1868 Q. Jrnl. Pure & Appl. Math. 9 320 The line joining it to a point exterior to the conicoid is the axis of the cone of which the focal point is the vertex. 1919 Ann. Math. 20 233 The four circumscribed circles are concurrent at a point. This will be called the focal point of the quadrilateral. 1964 R. K. Luneburg Math. Theory Optics 170 It follows that the point x = y = 0 is a common focal point of all the conics. 2003 H. Schaub & J. L. Junkins Analyt. Mech. Space Syst. ix. 383 The sum of the two radial distances from any point on the ellipse to each focal point is constant. This entry has been updated (OED Third Edition, September 2016; most recently modified version published online March 2022). < n.1692
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