ABSTRACT
In this chapter, the author reviews some of the important evidence relating to the propagation of light, both in empty space and in transparent material media. The propagation of the light involves the transport of the energy away from a source. The simplest picture of this process is in terms of a stream of particles emitted from the source; Pythagoras, back in the 6th century b.c., proposed this mechanism. It accounts, very directly, for the propagation of light in straight Unes (as evidenced by the sharpness of the shadows) and for the fact that light can travel with complete ease through a vacuum. Until about 1850 the propagation of light was envisaged in the purely mechanical terms. This, however, posed very considerable difficulties, because it was hard to understand how the speed of the light could be so very great.
