The anisotropy of mesogenic molecules, along with the high degree of orientational order in the liquid crystal phases, leads to several interesting optical phenomena. Light passing through a liquid crystal sample will experience different refractive indices depending on the polarization direction relative to the local director orientation. This means that the component polarized parallell to the director will traverse the medium with a different speed from that of the perpendicularly polarized component. This affects the phase relationship between the components and leads to a change in polarization state. For instance, if the incident beam is linearly polarized the polarization direction is generally changed on passage through the sample, and the light may in addition become eliptically polarized (see figure 5).
|Figure 5. When light passes through a liquid crystal cell it may change its state of polarization.|
When looking at liquid crystals in a microscope you often insert the sample between crossed polarizers. The first (often referred to as the polarizer) secures that the incident light is linearly polarized in one specific direction. Since the polarizers are crossed, no light would emerge through the second polarizer if the liquid crystal were absent. But when the light passes through the anisotropic sample its polarization state is altered and thus some of it may pass through the second polarizer (often referred to as the analyzer). The sample now often appears in bright colors, since the effect is wavelength dependent (the phase difference between the two components after passage of the sample is a function of the wavelength of the light). The beautiful pictures in our art gallery are taken in this way.