This set of experiments helped me to investigate the concepts of the indices of refraction of light and light scattering. To do this, I used a polymer dispersed liquid crystal (PDLC) display. A PDLC has two indices of refraction making it a birefringent material. Light polarized perpendicular to the director in a liquid crystal travels at a different speed than that polarized parallel to the director. The result is that the light waves emerging from the material have a phase difference. The fact that the light can travel at two different speeds in the liquid crystal means that the material is birefringent, or that it has two indices of refraction (See Figure 1).

To make a PDLC display, liquid crystal material and a polymer are mixed and placed between two conducting glass slides. As the polymer and liquid crystal solutions are combined, the mixture first appears clear. With time, the solution becomes opaque. This occurs because the long chains of the polymer and the liquid crystal molecules begin to separate. The liquid crystal molecules form into small droplets because they are becoming insoluble with the polymer molecules. The opaque appearance of the mixture stems from the fact that the liquid crystal and the polymer have two different indices of refraction (See Figure 2).

When a light wave encounters a molecule some of the wave's energy is transferred to the electrons of the molecule causing them to oscillate. These electrons emit light waves in all directions. If all of the molecules in a material are similar, the emitted waves will combine with the incident wave so as to produce a wave that travels slower than the original wave. The resultant speed depends on the index of refraction of the material, n, which can be characterized by both the polarization of the light and by its propagation direction. If the speed of the light depends on the polarization of the light, as discussed above, then the material is said to be birefringent.

If all the molecules in the material are not similar or are not arranged in a regular array, then the light radiated by the oscillating electrons does not recombine with the original wave. The emitted light is scattered throughout the material. This phenomenon gives materials a cloudy or opaque appearance. To test this theory, I performed an experiment dealing with the Tyndall Effect where light scattering particles are added to an optically clear (no light scattering properties) medium. The result is the creation of a medium which has light scattering properties.

I filled a large glass container with water and shined a flashlight beam through the container. When I looked perpendicular to the beam of light I could not see the light traveling through the water. (See Figure 3(a) ). The water in the container did not scatter the light sideways and the beam could not be seen. I then added some milk to the water. Proteins and fats are molecules which have light scattering properties. Because the water molecules were no longer surrounded by like molecules, the radiated light from the oscillating electrons did not recombine with the incident flashlight beam. The emitted waves were scattered, making the beam visible in a direction perpendicular to the incident beam. (See Figure 3(b) ). The white appearance of the material in the container was due to the fact that the milk was becoming equally distributed throughout the water and the emitted waves were being scattered nearly equally throughout the mixture.

This simple experiment lends itself nicely to an explanation of the operation of a PDLC display. As previously discussed, a PDLC display consists of liquid crystal droplets dispersed throughout a polymer layer. The LC droplets are birefringent having two indices of refraction depending on the polarization of the incoming light. The polymer, however, has only one index of refraction and is said to be isotropic. Near the surface of the droplets of liquid crystal, the LC molecules are surrounded by dissimilar molecules of the polymer. The result is a scattering of the incoming light by the droplets. The appearance of the display is a "milky" white. When the display appears opaque it is said to be in the "off" state (See figure 4).

If an electric field is applied across the PDLC then the droplets rotate so that their symmetry axes are aligned parallel to the field. The incoming light now only experiences the perpendicular index of refraction of the LC. If this index of refraction is similar to that of the polymer, the light scattering effect of the droplets is minimized. Now the display appears clear. When the display appears clear it is said to be in the "on" state (See Figure 4).

To test this theory I applied an AC voltage across a PDLC display. As the voltage was increased, the display became increasingly clear. Opositely, as the voltage was decreased, the display became increasingly cloudy. I also tried heating the display using the light of an overhead projector. As the liquid crystal display was heated, the display became clear. The molecules in the LC became disordered until the isotropic phase was reached. In this state, the birefringence of the LC was lost leaving only one index of refraction. The scattering properties of the droplets were at a minimum and the display appeared clear.

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