Reflection.—To explain reflection, it is supposed that the reflector repels the particles as they approach it, and so the path of one particle would be like that indicated by the dotted line in the diagram (Fig. 3).
FIG. 3.
Until reaching the point A we suppose that the particle does not feel appreciably the repulsion of the surface. After A the repulsion bends the path of the particle round until B is reached, and after B the repulsion becomes inappreciable again. The effect is the same as a perfectly elastic ball bouncing on a perfectly smooth surface, and consequently the angle to the surface at which the corpuscle comes up is equal to the angle at which it departs.
Refraction.—To explain refraction, it is supposed that when the corpuscle comes very close to the surface of the transparent substance it is attracted by the denser substance, e.g. glass, more than by the lighter substance, e.g. air. Thus a particle moving along the dotted line in air (Fig. 4) would reach the point A before the attraction becomes appreciable, and therefore would be moving in a straight line. Between A and B the attraction of the glass will be felt and will therefore pull the particle round in the path indicated. Beyond B, the attraction again becomes inappreciable, because the glass will attract the particle equally in all directions, and therefore the path will again become a straight line. We notice that by this process the direction of the path has become more nearly normal to the surface, and this is as it should be. Further, by treating the angles between the two paths and the normal mathematically we may deduce the laws of refraction which have been obtained experimentally. One other important point should be noticed. Since the surface has been attracting the particle between A and B the speed of the particle will be greater in the glass than in the air.
FIG. 4
Ejection and Refraction at the same Surface.—A difficulty very soon arises from the fact that at nearly all transparent surfaces some light is reflected and some refracted. How can the same surface sometimes repel and sometimes attract a corpuscle? Newton surmounted this difficulty by attributing a polarity to each particle, so that one end was repelled and the other attracted by the reflecting and refracting surface. Thus, whether a particle was reflected or refracted depended simply upon which end happened to be foremost at the time. By attributing suitable characteristics to the corpuscles, Newton with his superhuman ingenuity was able to account for all the known facts, and as the corpuscles were so small that direct observation was impossible, and as Newton's authority was so great, there was no one to say him nay.