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LENSES AND MIRRORS 137
no area), all rays reflected by the mirror surface would focus at point C. But a light source is not a point; it has considerable area. Therefore rays from its outer margins meet the mirror surface at a slightly different angle than those coming from its center. This has the effect of producing what we call "spherical aberration," since the difference of angle of incidence will cause rays striking the outer zones of the collector to focus closer than the rays from the central zones. If you lay the lines out on paper you will see that while the rays from the outer margins of the light source to the center of the mirror or lens will be affected, still the rays to the outer zones of the collector will be affected very much more. It is wholly a matter of the angles at which rays from the light source meet the different zones of the mirror surface.
The angle of incidence is always exactly equal to the angle of reflection. In a spherical mirror all points of the surface cannot correspond precisely at the same angle to points on a light source and therefore the rays reflected by the different zones of the mirror will focus at different distances from its reflecting surface.
(146) This condition causes spherical aberration. It can be corrected, partly, by tilting the outer zones of the mirror slightly backward which gives either a parabolic or an elliptical effect.
(147) It is evident that if light source A, in Fig. 47, was moved closer to the mirror, to B, the angles of incidence and reflection would be so altered that the beam would no longer focus at C, but much further back. It is this action that explains the reduction of enlargement of the spot diameter as the light source is moved nearer to or further away from the mirror.
Selection of Mirror Collector
(148) Silver has the highest power of reflection, and polished glass the smoothest, closest grain surface of any substances suitable for mirrors. Therefore they yield the highest possible accuracy and optical efficiency in any product designed for the reflection of light.