A New Optical Property of Biaxial Crystals

C. G. Raman and V. S. Tamma. On a new optical property of biaxial crystals. Philosophical Magazine, 43, p. 510, 1922.

The authors have discovered that a plane-parallel plate cut from an aragonite crystal perpendicular to the bisectrix of the acute angle formed by the optical axes of the crystal has the property of focusing, at a distance, the image of a point source.

The image is sharp, bright, and practically achromatic if the observation is made a few centimeters from the plate and the light source is brought close to it. The image remains sharp and bright even at a distance of 30–40 cm from the plate, but is decomposed into a spectrum. The phenomenon is especially effective if the light source is a mercury lamp. In this case three separate images are obtained: yellow, green, and violet. Unlike the action of a lens, the aragonite plate always gives a real, upright, unmagnified image, and moreover not in any single definite plane, but continuously over a fairly considerable distance.

The qualitative explanation proposed by the authors is as follows. The front of a wave propagating in the crystal is Fresnel’s surface, which has a singular point. This singular point serves as the point of intersection of those plane waves whose normals form the surface of a cone. The brightness of the singular point must therefore be very great in comparison with the other points of the wavefront envelope of the plane waves. On emerging from the crystal, the singular point is preserved. In its immediate vicinity the wavefront is approximately symmetrical with respect to the axis of the cone of normals; therefore the propagating wave preserves that singular point in which the energy of the light is concentrated and, in this way, the image of the source will be continuously transferred to a distance.

The authors propose in the near future to give a quantitative theory of the phenomenon.

S. Vavilov.