Chapter 1 alreadyGeometrical optics (*) described the everyday observation of sharp shadow casts. The wavefronts are planes, their normal vectors define the direction of propagation. If this is the case for spatially extended light bundles, we speak of parallel light. When a parallel light bundle illuminates an opaque object (here the boundaries of the aperture), a sharp shadow is cast. This supports the model idea ofGeometrical optics geometrical optics: lightGeometrical opticsrectilinear propagation propagates in a straight line. In this respect, light is represented by light rays, which in representations almost always propagate from left to right. The direction of propagation is occasionally indicated by arrows.