POLARIZATION OF LIGHT
One characteristic common to all transverse waves is polarization. For an electromagnetic wave, the direction of polarization is always defined as the direction of the electric field vector, and not the direction of the magnetic field. This is due to the fact that most electromagnetic wave detectors respond to the electric forces on electrons in materials and not on the magnetic forces.
For some electromagnetic waves such as radio waves, waves are said to be linearly polarized, that is, the waves may be vertically or horizontally polarized depending on the direction the electric fields’ oscillations. But for the electromagnetic waves emitted by visible light, the waves are said to be not polarized or in other terms, polarized randomly. Yes, a light source may contain a large number of molecules of different orientation, thus, the wave emitted by the source may be linearly polarized but changes direction randomly. This light is called unpolarized light.
Unpolarized light may be transformed to a polarized light by sending the light into a polarizing sheet or polarizing filter.
Light waves are polarized randomly or unpolarized since the electric field at any given point is always perpendicular to the direction of travel of the waves but changes directions randomly. Because of this, it is hard to draw a simple representation of a head-on view of the oscillations over a period of time. Thus, we need to resolve each electric field into y and z components to have a simplified image. See figure below.
The unpolarized light can be represented with a pair of double arrows. The oscillations of the net y component of the electric field are represented by the double arrow along the y-axis while the double arrow along the z-axis represents the oscillations of the net z component of the electric field. Through this process, the unpolarized light is effectively changed into the superposition of two polarized waves. The planes of oscillation of these waves are perpendicular to each other – one plane contains the y-axis and the other contains the z-axis.
A polarizing sheet is made up of certain long molecules embedded in plastic, in which it is stretched so that the molecules will be aligned in parallel rows. By sending an unpolarized light through a polarizing sheet, it can be transformed into a polarized light. The electric field components along one direction of the unpolarized light pass through the sheet. On the other hand, the components perpendicular to the direction of the electric field components are absorbed by the molecules and disappear. Instead of using molecules, a polarizing direction along which electric field components are passed is assigned to the sheet, thus, it can be stated that An electric field component parallel to the polarizing direction is transmitted by a polarizing sheet while the electric field component perpendicular to the polarizing direction is absorbed. This means that the unpolarized light, after passing through the filter, is polarized in the direction of the polarizing direction. See the figure below as an example.
Polaroids or Polaroid filters, invented by Edwin Land in 1932 are commercially known polarizing sheets.