Polarization of Light

Polarization of Light We have discussed in our previous lectures that Light wave consists of perpendicularly oscillating electric and magnetic fields. Both these vectors are mutually perpendicular to the direction of propagation of wave. Thus if the wave vector, k, points in the direction of propagation of wave, then: k=BxE Here k, B and E are vectors and by the property of cross product they are mutually perpendicular to each other. Therefore, if the electric field’s vector points along the x‐axis and the magnetic field vector along the y‐axis then wave vector points along the z‐axis. k E B However, you can have another set of vectors that satisfy the same cross product i.e. the set in which all vectors are rotated at some angle wrt to the original vectors about the direction of propagation. B B’ E E’ Here, the wave vector k is pointing out of the page. Therefore, there are infinite numbers of possible orientations in which the cross product relation can be satisfied. In typical common sources of light this is exactly the case i.e. the electric and magnetic field vectors vibrate along planes that is randomly distributed about the direction of propagation. Such a form of light is called unpolarized light. We define the plane of electric field to be the plane of polarization of light. When the electric field vector oscillates in a single plane, the light is said to be have been linearly polarized, or simply polarized. Light can be polarized in two different ways: Polarization by absorption and polarization by reflection. Lets look at each of these methods separately. Polarization by Reflection A polarizer or polariser is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well‐defined polarization, polarized light. The resultant plane of polarization of light that emerges from a polarizer depends upon the plane of polarization of the polarizer. Only that light passes through the polarizer whose plane of polarization is parallel to the plane of the polarizer. A Polaroid polarizing filter is made from polyvinyl alcohol (PVA) plastic with an iodine doping. While the sheet is being stretched the PVA chains align in one particular direction. Valence electrons from the iodine doping can move linearly along the polymer chains, but not transverse to them. So incident light polarized parallel to the chain is absorbed in the sheet while the light polarized perpendicularly to the chains is transmitted When a single polarizer is placed in the path of an unpolarized light, the emergent light only oscillates in a single planed, parallel to the plane of polarization of the polarizer. If another polarizer is placed in the path of polarized light whose planes are perpendicular to each other, all the light is absorbed in the polarizer since there is no light component oscillating parallel to the plane of the polarizer (as shown in the picture above). This second polarizer is often called the analyzer. Polarization by Reflection Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces. The extent to which polarization occurs is dependent upon the angle at which the light approaches the surface and upon the material that the surface is made of. The angle of incidence at which a surface polarizes all the reflected light is called the materials Brewster Angle. At this angle the reflected and refracted light are at 90 degrees to each other. For glass, the Brewster angle is approximately 55 degrees.