A first‐motion alternative to Geometrical Ray Theory

An alternative to geometrical ray theory is obtained. This first‐motion approximation is more generally valid than geometrical ray theory. The approximation had previously been obtained using the generalized and quantized ray theories, but can also be derived from the more usual wavenumber‐frequency integral method. The first‐motion approximation is useful not only when geometrical ray theory is valid, but also at caustics and shadows.