In this set of experiments, my first priority was to review the concepts of polarization. In the case of unpolarized light, all directions of the electric field vector are equally possible. All the electric field vectors are perpendicular to the direction of propagation (as shown in Figure 1 (a) below).

If the light wave is linearly polarized, however, E vibrates in the same direction perpendicular to the direction of propagation at all times. This is also known as "plane polarization" or just "polarization". This case is shown above in Figure 1 (b). In order to achieve linearly polarized light, unpolarized light is passed through a polarizing filter. Only the light waves which vibrate in a direction parallel to the direction of polarization are allowed to pass through the filter (as shown below in Figure 2).

As is shown in the diagram, light will pass through two polarizing filters with their polarizing axes in the same direction. No light passes through two polarizers with their polarizing axes perpendicular. In this case, the polarizers are said to be "crossed".

In the nematic phase of liquid crystals, the molecules can move freely in space, but there exists an orientational order in the liquid crystal that causes the molecules to align with one another. Liquid crystal molecules are considerably longer than they are wide. Intermolecular forces cause these molecules to align with their long axes parallel to one another. This alignment is known as the orientational order. The molecules align with what is known as the director. Liquids do not possess any orientational order. Therefore, the orientation of the molecules in a liquid are random. (See Figure 3)

In the TN display, nematic liquid crystal molecules are placed between two plates of glass with microgrooves at right angles to each other. The molecules experience a twist as shown in the figure below. When light travels through the display, it follows the twisted orientation of the molecules. This is the reason that we observe transmittance when we place a TN LCD between crossed polarizers (see Figure 4).

To demonstrate the concepts just discussed, I viewed a twisted nematic, (TN), liquid crystal display through a polarizing filter. In order to obtain a TN LCD, you can remove such a display from an ordinary wrist watch or calculator. I wrote to ALCOM and they sent me a TN LCD, so I didn't have to do this. Once the display has been acquired, to turn it "on" I used both static electricity and a 9-volt battery. To use static electricity, I had someone hold the bottom glass plate of the display while I rubbed my feet on the floor to create a static charge. I then ran my finger along the display. Portions of the display switched "on" by turning dark. To use a 9-volt battery, I connected a wire to one of the battery terminals and to one end of the bottom plate on the display. Then I connected a second wire to the other battery terminal and ran the end of the wire along the electrodes on the bottom glass plate. This technique rendered the same effects as the static electricity.

With the display in the on state, I looked at it through a polarizing filter. Rotating the filter through 90 degrees I could see the display turn from bright to dark. This occurred because there is linearly polarized light exiting the display. When I rotated a polarizer through 90 degrees, I slowly reduced the amount of light that was parallel to the axis of polarization until the light was perpendicular to the axis of polarization. This led to no light transmission as shown in Figure 2 above.

In addition to viewing the TN LCD through the polarizer, I also experimented with the effects of heat on the display, by holding the display up to the light of an overhead projector. The display was initially a silvery surface. The silvery appearance stems from the loss of intensity of the outcoming light as a result of the crossed polarizers in the display and from the reflecting mirror at the back of the display. Upon heating the display I observed that it became dark as if no light was being transmitted through the display. In effect this is what was happening. By heating the display, the liquid crystals went from the twisted nematic state (as shown in Figure 4) to the isotropic state. In the isotropic state, the molecules of the liquid crystal become disordered making all directions in space equivalent. The liquid crystal is now simply a liquid. It can no longer rotate light, therefore the crossed polarizers do not allow any transmission to the reflecting mirror and the display appears dark. When I let the display cool the silver appearance returned. As the display cools, order is regained and the liquid crystal can again rotate light allowing transmission to the reflecting mirror.

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