Polarization

Light, an electromagnetic wave, is said to be plane polarized if the electric field of the light always lies in the same plane.

Most light around us is not polarized. Unpolarized light can be polarized by passing through a polarizer. A polarizer is a sheet of material with a molecular structure that only allows a specific orientation of the electric field to go through. When unpolarized light passes through a polarizer, the intensity of the light is halved.

Analyzer. An analyzer is a polarizer used to determine if light is polarized. A polarizer reduces the intensity of unpolarized light by 50%, no matter the orientation of the polarizer. Yet if the light is already polarized, then a polarizer (analyzer) will reduce the intensity differently depending on the angle of orientation.

Malus's Law: ... I = I0 cos2(theta). This equation models how the polarizer reduces the intensity of the transmitted polarized light. Theta is the difference in angles between subsequent filters. I0 must be polarized light. "I" is the intensity of light that leaves the second polarizer (analyzer).

 

Polarization by reflection. When unpolarized light reflects off a non-metallic surface, the reflected ray is partially polarized. The plane of polarization is parallel to the reflecting surface.

 

Brewster's Angle. The degree to which the light is polarized depends on the angle of incidence. When the reflected ray is 100% polarized, the angle between the reflected ray and the refracted ray is 90 degrees. This combination can be determined from Snell's Law as dictated by the indices of refraction (or speed of light in each medium).

Tan (theta)B = n2/n1.

 

Optical Activity. Solutions that rotate the plane of polarization are said to be optically active. Sugar solutions, tartaric acid and turpentine, as well as crystals. Angle of rotation is determined by the wavelength of light and the distance the light travels through the substance. Some substance rotate clockwise and others counterclockwise.

 

Stress Analysis. Complicated patterns are seen on certain materials that are under stress when placed in between two polarizers that are at right angles to each other. Variations in the pattern will help determine how the stress varies in the material.

LCDs. Liquid crystals are placed between two polarizing plates with orientation 90 degrees to each other. Liquid crystals between plates change orientation as they progress to each plate. Therefore light will travel through the plates as it changes orientation along with the crystals. Yet, if a potential difference is established between the two glass plates the molecules will align with the electric field and will not change orienation. Therefore the light will not be able to pass through the back polarizer.

If voltage is applied to particular pixels on the screen we will be able to determine which pixels are lit up and which remain dark. Your TI-84 is a simple example of this.

PG 278: 3-16, 18-20, 22.

 

 

 

 

 

 

 

circular and 3-D movies http://www.lhup.edu/~dsimanek/14/polaroid-short.htm

circular polarizers http://en.wikipedia.org/wiki/Polarizer#Circular_polarizers