The vibrations of the ether that produce a ray of light are transverse vibrations in every direction. Figure 1 may be taken to represent a cross section of a ray of light, showing vibrations in a few of these directions. If all the vibrations were parallel transversely, as in Fig. 2, then the ray would be a ray of plane polarized light. Polarized light affects the eye just like ordinary light, but it presents certain very interesting phenomena, some of which are shown in the following pages.
The crystal tourmaline has the property of permitting vibrations in only one plane to emerge from it; that is, it polarizes light.
Demonstrations. - Place two tourmaline crystals one over the other and parallel to each other. The light that passes through one will pass through the other. Now turn one of them until it crosses the other at right angles, as in Fig. 3, and no light at all passes through the crossed portion. The action is as though each let through only those vibrations which are parallel to its length. Now put between the tourmalines a piece of quartz or Iceland spar, and on turning one of the tourmalines, brilliant color effects are observed. The effect of the quartz, then, must be to turn the plane of polarization and to enable a part of the light to pass through the second tourmaline, producing the color effects by partial interference of the polarized rays.
Lay a sheet of black paper upon the table. Over this lay a sheet of glass G (Fig. 4). Cut out ten or twelve pieces of thin glass, and holding them as at A in the figure, look through them at a piece of mica M laid upon the glass sheet. Hold the thin glasses at various angles and elevations, and determine the position in which the most brilliant effects are produced. The same results are obtained if the eye is placed below the glasses in such a position that the polarized ray is reflected from B.
