. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. Chap, xxiv.] REAL AND VIRTUAL Foci. 305 meet in the principal focus. Similarly, the more f approaches the mirror the smaller angle do its rays make with the normal, the smaller, therefore, grows the angle of reflection, and the more does F' approach to c. When f is at c, its rays are normal to the sur- face ; they are reflected in the same line, and the source of light and the focus coincide. Real and virtual. foci.—In all the cases
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. Chap, xxiv.] REAL AND VIRTUAL Foci. 305 meet in the principal focus. Similarly, the more f approaches the mirror the smaller angle do its rays make with the normal, the smaller, therefore, grows the angle of reflection, and the more does F' approach to c. When f is at c, its rays are normal to the sur- face ; they are reflected in the same line, and the source of light and the focus coincide. Real and virtual. foci.—In all the cases that have been considered the source of light is not nearer the mirror than the principal focus, and the principal and conjugate foci liave all been on the same side of the mirror as the source of light. They are, therefore, called real foci. When, however, the source of light is nearer the mirror than the principal focus, the angle of incidence is so great that the reflected rays become divergent from the axis. Thus, in Fig. 181, AB is again the mirror, and the other letters are also the same as before, f is the source of light, /A /B are the in- cident, and AM BN the reflected rays. Being divergent, the re- flected rays cannot meet on the same side of the mirror as this source of light, but if prolonged backwards they meet in a point F , which IS a Virtual foCUS, because of Concave Mirror. it is not on the same side of the mirror as the source of light. In convex mirrors the foci are always virtual. The principal focus (virtual) is formed by letting parallel rays fall upon the mirror. The reflected rays diverge, but if prolonged backwards meet in a point on the prolongation of the principal axis. That point is the principal virtual focus, and gives the principal focal distance, equal to half the radius of curvature. As in concave mirrors, if the rays falling on the mirror be divergent, they form a conjugate focus, u—7. Please note that these images a
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