Crystal Optics for Visible Light and X Rays

This summary of much of the author's work stresses the unity of approach to the understanding of the optical properties of a crystalline medium for visible light as well as for x rays. The scattering elements of the crystal are supposed to act as dipoles, and for the interior of the medium the problem is that of finding a balanced, or self-consistent state of optical field and dipole vibration. This is an eigenvalue problem, with the phase velocity as the eigenvalue. Its solution leads on to finding the forced vibrations in a crystal with a plane surface on which an external plane wave falls, and gives the theory of reflection and refraction. The physical interpretation of the various mathematical steps of the theory is stressed. It leads, as an example, to an immediate qualitative understanding of the anomalous absorption occurring for x rays near the Bragg angles (Borrmann effect). A mechanical model of this effect is discussed.