Polarization is a phenomenon peculiar to transverse waves, i.e., waves that vibrate in a direction perpendicular to their direction of propagation. Light is a transverse electromagnetic wave (see electromagnetic radiation). Thus a light wave traveling forward can vibrate up and down (in the vertical plane), from side to side (in the horizontal plane), or in an intermediate direction. Ordinarily a ray of light consists of a mixture of waves vibrating in all the directions perpendicular to its line of propagation. If for some reason the vibration remains constant in direction, the light is said to be polarized.
It is found, for example, that reflected light is always polarized to some extent. Light can also be polarized by double refraction. Any transparent substance has the property of refracting or bending a ray of light that enters it from outside. Certain crystals, however, such as calcite (Iceland spar), have the property of refracting unpolarized incident light in two different directions, thus splitting an incident ray into two rays. It is found that the two refracted rays (the ordinary ray and the extraordinary ray) are both polarized and that their directions of polarization are perpendicular to each other. This occurs because the speed of the light in the crystal—hence the angle at which the light is refracted—varies with the direction of polarization. Unpolarized incident light can be regarded as a mixture of two different polarization states separated into two components by the crystal. (In most substances the speed of light is the same for all directions of polarization, and no separation occurs.)
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