As described in Chapter 4, while traversing a birefringent material, the different speeds of the ordinary and extraordinary waves engender a phase retardation within the emerging light that alters its polarization state. If that phase retardation can be controlled, in conjunction with a second polarizer (the analyzer), the intensity of the light can be modulated by the altered polarization state. This is the essence of the operation of a liquid crystal display.
In preview, the long rod-shaped molecules of a liquid crystal provide sufficient anisotropic structure to have fast and slow axes, and linearly polarized light passing through, depending on the angle of incidence, can be decomposed into ordinary and extraordinary waves, engendering phase retardation that causes changes in the polarization state of that light just as in the solid crystal case described above. But a liquid crystal goes one better: The molecular orientation of the liquid crystal is easily changed, owing to its inherent fluidity, and thus its optical birefringence (derived from its dielectric anisotropy) is also easily changeable, and since an external electric field can control the birefringence, the polarization state of the light passing through can be precisely controlled, and in conjunction with a second polarizer, the intensity of light can be modulated.
The first recorded observation of a liquid crystal was in 1888 by the Austrian botanist Friedrich Reinitzer. In his investigation ...