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 a crystal of aragonite 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 distinct, bright, and practically achromatic if observation is made a few centimeters from the plate and the source of light is brought close to it. The image remains distinct 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 a mercury lamp serves as the light source. 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, erect, unmagnified image, and moreover not in any definite plane, but continuously over a rather considerable distance.

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

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

S. Vavilov.