Polarization of Light Waves

Polarization is a characteristic of all transverse waves.

An ordinary beam of light consists of a large number of waves emitted by the atoms of the light source. Each atom produces a wave having some particular orientation of the electric field vector , corresponding to the direction of atomic vibration. The direction of polarization of each individual wave is defined to be the direction in which the electric field is vibrating. In Figure 3.3, this direction happens to lie along the y axis. However, an individual electromagnetic wave could have its vector in the yz plane, making any possible angle with the y axis. Because all directions of vibration from a wave source are possible, the resultant electromagnetic wave is a superposition of waves vibrating in many different directions. The result is an unpolarized light beam, represented in Figure 3.4a. The direction of wave propagation in this figure is perpendicular to the page. The arrows show a few possible directions of the electric field vectors for the individual waves making up the resultant beam. At any given point and at some instant of time, all these individual electric field vectors add to give one resultant electric field vector.

Figure 3.3. Schematic diagram of an electromagnetic wave propagating at velocity c in the x direction. The electric field vibrates in the xy plane, and the magnetic field vibrates in the xz plane.

Figure 3.4. (a) A representation of an unpolarized light beam viewed along the direction of propagation (perpendicular to the page). The transverse electric field can vibrate in any direction in the plane of thepage with equal probability. (b)A linearly polarized light beam with the electric field vibrating in the vertical direction.

A wave is said to be linearly polarized if the resultant electric field vibrates in the same direction at all times at a particular point. (Sometimes, such a wave is described as plane-polarized, or simply polarized.) The plane formed by and the direction of propagation is called the plane of polarization of the wave. If the wave in Figure 3.3 represents the resultant of all individual waves, the plane of polarization is the xy plane.

We always define the direction of polarization of an electromagnetic wave to be the direction of the electric-field vector not the magnetic field, because many common electromagnetic-wave detectors respond to the electric forces on electrons in materials, not the magnetic forces.

It is possible to transform unpolarized light into polarized light. Polarized light waves are light waves in which the vibrations occur in a single plane. The process of transforming unpolarized light into polarized light is known as polarization. There are a variety of methods of polarizing light. The four methods discussed on this page are:

· Polarization by Transmission

· Polarization by Reflection

· Polarization by Refraction

· Polarization by Scattering.

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