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Let us recount some of the characteristics of the motion of particles and the propagation of waves, with a view toward analyzing the behavior of light. In accordance with Newton's first law, a particle moves in a straight-line path in the absence of external forces. Thus we might infer, as Newton suggested, that light is composed of particles, and that, in a continuous medium, there is no deflecting force on the light particles. At the interface between two media, light may be propagated in a straight line parallel to the interface. Thus even at an interface there is no force on the particles of light unless the light passes through the interface, and in that event the force acting must be perpendicular to the interface. To account for the fact that light is refracted toward the normal on passing from a rare to a dense medium, as from air to water as shown in Figure 40-1, it is necessary to assume that the force is directed from the rare to the dense medium. The normal component of the velocity is increased, while the tangential component remains the same, so that the refracted ray is more nearly directed toward the normal. From this we see that a particle theory of light implies that the velocity of light in the dense medium is greater than the velocity of light in the rare medium.