Focus

(fo'küs,) (Eng. pl. FoCUSES; L. FOCI.) [L., a fireplace.] (Math.) In Geometry and Conic Sections, the F. of a parabola is a point in the axis having this property, that a radius drawn from it to any point in the curve makes the same angle with the tangent, at that point, that the tangent makes with the axis. Hence, if parallel rays of light fall on the parabola, they are reflected to the F.; or rays emitted from the F.will be reflected in a direction parallel to the axis. The foci of an ellipse are situated in the major axis, at equal distances from the centre; and the sum of two straight lines drawn from them to any point in the curve, is with the same ellipse always the same quantity. Also these two lines make equal angles with the tangent at that point. Hence rays of light, ., emitted from one F. are reflected to the other. The foci of an hyperbola are also in the major axis, at equal distances from the centre; but the difference between two straight lines drawn from them to any point in the curve, is with the same hyperbola always the same quantity; and these two lines make equal angles with the tangent at that point, but at opposite sides of the curve. Hence aray of light emitted from one F. will be reflected into the direction of a ray coming from the other; or rays pa ing towards one F. will be reflected to the other.(Opt.) The point of convergence at which all the rays of light meet after pa ing through a convex lens. It should be observed, however, that the F. is not, strictly speaking, a point, but a small circle, which bears the same relation to the apparent diameter of the lens that the image of any other object, formed in the F. of the lens or mirror, bears to the object itself. Only a point can give the image of a point. [s. 372]