Cover: Propagation of rays of sound waves through a cross section of a solar model (see also Section 5.2.3). The ray paths are bent by the increase with depth in sound speed until they reach the inner turning point (indicated by the dotted circles), where the waves undergo total internal refraction. At the surface the waves are reflected by the rapid decrease in density. The rays correspond to modes with frequency of 3000 microHz; in order of decreasing depth of penetration their degrees l$ are: 0 (the straight ray passing through the centre), 2, 20, 25 and 75.