Wednesday 8-7-96
The relevant section in the book is 23.4
Light, as we talked about before, is simply an electromagnetic wave, with an electric field oscillating one way and a magnetic field oscillating perpendicular to the electric field. Both fields are perpendicular to the direction the light is travelling. Light is produced by electrons in atoms dropping down from one energy level to another. Each time this happens, the electric and magnetic fields will have a certain orientation. Taken together, though, the fields are randomly oriented, and the light is said to be unpolarized. Most light sources emit unpolarized light.
If the electric fields all line up (which means the magnetic fields all line up too), however, the light is polarized. With linearly polarized light, the light has its electric field vectors all pointing the same way, and they stay in that orientation as the light travels.
Light can be polarized in several ways, one way being by reflection. If a light beam strikes an interface so that there is a 90 degree angle between the reflected and refracted beams, the reflected beam will be linearly polarized, with the direction of polarization (the way the electric field vectors point) being parallel to the plane of the interface.
The special angle of incidence that satisfies this condition, where the reflected and refracted beams are perpendicular to each other, is known as the Brewster angle. If light is coming from one medium with index of refraction n1, and striking a medium with an index of refraction n2, the Brewster angle is given by:
Brewster angle, tan theta = n2 / n1
There are plenty of materials that affect the polarization of light. Certain materials (such as calcite) exhibit a property known as birefringence, which stems from the fact that light travels at different speeds through a birefringent material depending on the direction it goes through the material. A crystal of birefringent material affects light polarized in a particular directiondifferently from light polarized at 90 degrees to that direction; it refracts light polarized one way at a different angle than it refracts light polarized the other way.
Another interesting material property related to polarization is dichroism. A dichroic material absorbs light of a particular polarization, while not absorbing light polarized perpendicular to that direction. If the material is thick enough to absorb all the light polarized in one direction, the light emerging from the material will be linearly polarized. Polarizers (such as the lenses of polarizing sunglasses) are made from such material.