Ordinary electromagnetic waves in optically biaxial crystals

It is shown that the so-called ordinary electromagnetic waves (the waves wh ose group velocity is parallel to the wave normal) cannot propagate outside the planes parallel to the principal planes of the permittivity tenser in transparent optically biaxial crystals. For each of the three such planes, the waves of one of the two independent branches are ordinary irrespectivel y of the direction of their propagation, whereas the. waves of the other br anch are ordinary only if they propagate along two mutually orthogonal dire ctions of the corresponding eigenvectors of the permittivity tenser. (C) 20 01 MAIK "Nauka/Interperiodica".