Polarisation

There are certain phenomena which offer conclusive proof that light has the characteristics of a wave motion in which waves vibrate in all possible planes at right angles to the direction of propagation. Such waves are called transverse.

The following experiment with two crystals of tourmaline is intended to make the nature of this phenomenon more evident.

When a crystal of tourmaline (Fig. 15) is cut parallel to the crystallographic axis and a ray of light is allowed to pass through it, the transmitted beam in no way differs from the incident beam, as far as the unaided eye can detect. If this is followed by allowing the fight to pass through another tourmaline crystal with its axis parallel to

Fig. 15 Polarization of light by tourmaline crystals.

that of the first, the rays will be almost completely transmitted by the second crystal as well. By rotating the second crystal about the ray of light as an axis so that the axes of the two crystals are inclined to each other, we will make the intensity of the transmitted light decrease until, when the axes of the crystals are at right angles to each other, none of the light from the first crystal will pass through the second one.

In case the rotation of the second crystal is continued until the axes of both are once more parallel, the light from the first one will again be transmitted by the second. It must be evident that in passing through the first crystal light has acquired some property ordinary light does not possess.

The action of the tourmaline will be understood if we consider ordinary light to consist of transverse waves in which vibrations take place in all directions in a plane normal to that of propagation.

When such a beam passes through the first tourmaline crystal, the latter absorbs all vibrations except those in a certain direction. There it transmits. Hence, the emerging beam differs from the ordinary light in that all the vibrations are but in one plane.

Such a ray of light is said to be polarized. If it falls on the second tourmaline crystal, the transmittance of the latter will, in its turn, be selective — i.e. it will transmit only the vibrations parallel to a certain direction in itself. Consequently, for one position of the crystals all the vibrations are transmitted. Whenever one of the crystals is rotated 90 degrees from this position, none of the vibrations are transmitted. It should be evident that for intermediate positions part of the vibrations are transmitted whereas the others are not.

A simple method of obtaining polarized light is by reflection from a plate of glass set at such an angle that the incident and reflected rays are perpendicular. In other words, this means that for the reflected beam to be completely polarized the angle of incidence must equal the arc tangent of the index of refraction of the glass, or

i = arctn n

where i is the incidence angle to be obtained for the total polarization and n is the index of refraction of the glass. For ordinary glass the angle i is about 57°. The amount of light reflected at this angle being small, it is customary to use a pile of glass plates when attempting to polarize light with this technique.