| 单词 | principal focus |
| 释义 | > as lemmasprincipal focus principal focus n. Optics the focus of rays that strike a lens or spherical mirror parallel to its axis. ΚΠ 1672 I. Newton in Philos. Trans. (Royal Soc.) 7 5085 This Table shews, how much the exterior rays, at several Apertures, fall short of their principal focus. a1764 J. Harris Treat. Optics (1775) i. 60 An object placed nearer a convex lens..than its principal focus..will have no real image. 1807 T. Young Course Lect. Nat. Philos. I. xxxv. 415 The focus into which parallel rays are collected, or from which they are made to diverge, is called the principal focus of a surface or substance. 1939 Brit. Jrnl. Psychol. 29 253 The light from an electric bulb, rendered nearly parallel by a condenser, filled a large lens at whose principal focus the eye was placed. 1985 R. Muncaster A-level Physics (ed. 2) xix. 314 The focal point (principal focus) of a converging lens is that point, on the principal axis of the lens, to which rays of light which are parallel and close to the axis converge after refraction at the surfaces of the lens. < as lemmas |
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