 So this is a cross-section through a homogenous sphere and let's just imagine that we have an earthquake that happens somewhere on the surface of the sphere, like over here. Now, the wave fronts will travel outwards from the source, sort of like this, you know, the way that noise travels in all direction from a noise source, or if you toss a rock in a pond, waves travel outward in concentric circles from the source. So these are kind of pictures of what the wave fronts look like. Now the ray paths are perpendicular to the wave fronts, so we draw them like this. And in a homogenous sphere, all the ray paths would just travel straight through the sphere to the other side, like this.