On Black Friday (in case you are wondering, the Friday immediately following Thanksgiving was given the label black because this first day of the holiday shopping season is often very profitable for merchants and traditionally merchants when recording losses and profits in their records would use red ink to indicate losses and black ink to indicate profits) I went with my grandmother to several electronics stores to pick out a new television for a small desk in her kitchen area. Because space is at a premium on her kitchen desk, I began leaning toward flat panel displays as the best option for her new television. However, as I began comparing different models and prices, I realized that I really had no idea how these extremely thin displays could produce a picture without a cathode ray tube (CRT) containing a scanning electron gun exciting phosphor atoms on the screen. Plasma and liquid crystal (LCD) displays accomplish the task of producing a picture on a screen in very different ways, but the ultimate product is a screen only several inches thick. Let us begin with LCDs, since they have been around longer, are more prevalent in electronic devices other than televisions, and is what I ultimately chose to get for my grandmother's kitchen desk.

Phases of Matter

The fundamental element of a LCD that gives it its name and ultimately makes it work is the liquid crystal. At first, this name seems to be a contradiction in terms. How can something simultaneously have the properties of both a liquid and a solid? We are all familiar with the common states of matter: solids, liquids, and gases. We might think of these states of matter as having properties related to the characteristics of the molecules which constitute a substance. That is, we can characterize a substance as one of these states of matter by looking at the properties of the molecules which make up the substance. Solids are extremely ordered substances, where molecules are arranged very neatly in latticelike structures. In this state, the molecules have definite position and orientation. Liquids, on the other hand, have neither definite position nor orientation. To elucidate this difference between liquids and solids, consider a box of the kid-favorite cereal rice krispies. In the box, these rice krispies are free to shift and rotate if the box is stirred or shaken. In the cereal box, the rice krispies act much like the molecules in a liquid. However, if you take those rice krispies out of the box and mix them with a melted marshmellow binder, you produce a rice krispie treat. In this state, each rice krispie is locked into a position and orientation by the marshmellow filler. In a rice krispie treat, the rice krispies are much like the molecules in a solid.

Microscopic View of a Liquid Crystal
Dr. Oleg Lavrentovich, Liquid Crystal Institute

What is a liquid crystal?

A liquid crystal is a phase of matter in between a solid and a liquid. Similar to a solid, the molecules in a liquid crystal tend to maintain their orientation, but similar to a liquid, they are free to change their position. At first, this seems quite odd. In a solid, the orientation of the molecules is set by the bonds which also establish their definite positions. Therefore, if in a liquid crystal molecules are free to change their positions, then the bonds between the molecules cannot be the source of the orientations. Instead, liquid crystals that have a definite order or pattern (referred to as the nematic phase of liquid crystals) have an orientation set by a director. This director can be many different things, including but not limited to an electric/magnetic field or (and more relevant to LCDs) a surface with microscopic grooves in it. It is important to point out that on average the orientations of the molecules of the liquid crystal are aligned with the director of the crystal, such that not all (and perhaps no molecules) will be perfectly aligned with the director. Below is a diagram of a nematic liquid crystal. Notice how only a few molecules are perfectly aligned with the director, labeled as the vector n, while most others are nearly aligned with the director.

Diagram of a Nematic Liquid Crystal
Credit: Liquid Crystal Research Group at University of Colorado.

Geometric Optics

Now that we have an idea of how liquid crystals are structured, let us investigate how this physical structure affects the optical properties of the crystal (how the crystal interacts with light). In general, light incident upon a barrier between two different materials (such as glass and air) will either be transmitted through the barrier (refraction) or bounce off of the barrier (reflection). The degree to which the light is reflected or refracted at the boundary is a function of what are called the indices of refraction of the two materials which meet at the boundary. The index of refraction of material i, n, is given by