Light; a course of experimental optics, chiefly with the lantern . cand A. If we take a divergent pencil as in Fig. 29, wefind still the same thing.: here c is still the centre ofthe mirror, from which all lines drawn to it are perpen-diculars to its surface. One such normal is shown by thedotted line, and simple inspection makes it evident thatthe lowest ray from s, reflected from the mirror at an II.] CONCAVE MIRRORS. 41 equal angle on the other side of that dotted line, pro-ceeds to s, and that every other ray from s is reflected tonearly the same point. The qualification nearly isnecessary
Light; a course of experimental optics, chiefly with the lantern . cand A. If we take a divergent pencil as in Fig. 29, wefind still the same thing.: here c is still the centre ofthe mirror, from which all lines drawn to it are perpen-diculars to its surface. One such normal is shown by thedotted line, and simple inspection makes it evident thatthe lowest ray from s, reflected from the mirror at an II.] CONCAVE MIRRORS. 41 equal angle on the other side of that dotted line, pro-ceeds to s, and that every other ray from s is reflected tonearly the same point. The qualification nearly isnecessary, because a carefully constructed mathematicaldiagram will show that only a parabolic form will exactlyconverge parallel rays to one point, and that a sphericalmirror only exactly converges rays emanating from the centreof the sphere. But for small surfaces the aberration is smalleven with spherical mirrors ; and these are the correct figurefor aiding the light in lanterns or electric cameras, the raysproceeding from the centre of curvature, and being reflected. Fig. 29.—Image formed by Concave Mirror. back through the same point, so as to reach the condensersat exactly the same angles as the direct light. Parabolicmirrors are often fitted by opticians to lanterns; but amoments reflection will show that such an arrangement isa mistake, as the condensers cannot deal properly at oncewith the divergent light from the radiant and the parallellight from the mirror. On the other hand, when a strongparallel beam is required, as in lighthouses, the parabolais the correct form. The point where parallel rays convergeis called the principal focus. 42 LIGHT. [CHAP. 24. Images from Concave Mirrors.—As the raysfrom one point are collected by a concave mirror to anotherpoint, as in Fig. 29, we must necessarily have an image(§12). And because a wide cone of rays is thus collectedand converged, this will be a brighter image than those pre-viously obtained. We have supposed s to be a point
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