Microscopy with Polarized Light

Abstract

The historic observation of double imaging by a highly birefringent crystal, such as that of calcite (see Chapter 1), can easily be repeated on an overhead projector. A small dot of black paper is pasted on the projector’s window near its center, then a cleavage rhombohedron of clear calcite is placed over the dot. Two images of the dot appear on the projection screen, which means that the incident beam is being divided into two beams that do not interfere with one another because they are vibrating in different (perpendicular) planes. When the calcite rhombohedron is rotated on whatever face it happens to rest, one image of the dot is stationary while the other image curiously traces a circle around the first image (see Figure 5.1). The fact that the so-called extraordinary image is displaced from the ordinary image means that the extraordinary ray travels at a different velocity than the ordinary ray. The fact that the extraordinary ray traces a circle rather than a spot confirms that the difference in velocities varies with the orientation of this crystalline species. The ordinary image is stationary because its velocity is constant with all orientations of the crystal, and accordingly the ordinary refractive index ω is constant. The various values for the refractive index in the path of the extraordinary ray can be given a general symbol, such as e′, or specific symbols, such as el, e2 e3, etc.