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The operation of liquid crystal displays is founded on the phenomenon of the double refraction of light as first recorded in Denmark by Erasmus Bartholinus in 1670. A piece of translucent calcite apparently divides incident light into two streams, producing a double image. This is depicted in Figure 1.1, as shown by the offset of the word âcalcite.â At about the same time in the Netherlands, Christian Huygens discovered that the light rays through the calcite could be extinguished by passing them through a second piece of calcite if that piece were rotated about the direction of the ray; this is depicted in Figure 1.2. This may be observed by taking two pairs of polarizing sunglasses and rotating them relative to each other.
One hundred and thirty-eight years later, in 1808, a protégé of the famous French mathematician Fourier, Etienne Louis Malus, observed that light reflected from a window, when passing through a piece of calcite also would change intensity as the calcite was rotated, apparently showing that reflected light was also altered in some way. The intensity of the light changed in both cases because the molecules of calcite have a crystal order that affects the light in an intricate but very understandable way called polarization.
It would be another 80 years later in Austria that double refraction, also called birefringence, and light polarization would be observed, not in crystalline rocks, but in a viscous liquid, later to be called a âliquid crystal.â Although no doubt intriguing to natural scientists, intensive investigation of liquid crystals had to wait for yet another 80 years, when commercial interests provided the impetus for further study.
Briefly, a liquid crystal display can reproduce an image of a scene through the use of a video camera that, upon receiving the light reflected from the scene through its lens, in accord with the photoelectric effect first explained by Einstein, an electric current is generated in a metal when struck by light of sufficient energy, the current being proportional to the intensity of that light. That current is then transmitted to transistors that control an analog voltage that is applied to a pair of transparent electrode plates. Those plates enclose a thin layer of liquid crystal between them, and the voltage on the plates generates an electric field that is used to control the orientation of the electric dipole moment of the liquid crystal molecules, causing them to turn. Then light from a light source placed behind the liquid crystal layer, after being linearly polarized by a polarizer, will have its polarization states altered by the different orientations of the liquid crystal molecules, in accord with the liquid crystalâs degree of birefringence. The beauty of the liquid crystal display is that the birefringence effected by a liquid crystal is precisely controllable by that electric field. The different polarization states of the light in conjunction with a second polarizer changes the brightness of the light emanating from the backlight source, and that modulated brightness can represent the light intensity of the original scene; the millions of picture elements so produced then combine to form an image that replicates the original scene.
Liquid crystal displays thus are based on an optical phenomena of electrically controlled birefringence and polarization, which can only be understood through knowledge of the interaction of light and matter.
However, light may be familiar to everybody, but Samuel Johnson succinctly observed that [1]*
We all know what light is, but it is not easy to tell what it is.
The understanding of light can gainfully begin at the outset with an appreciation of light as described by the Maxwell equations.
Note
* Samuel Johnson (1709â1784), English lexicographer, critic, poet, and moralist who completed the Dictionary of the English Language in 1755; Johnson is one of the preeminent authorities on the English language.
Reference
[1] Johnson, S. 1755. Boswellâs Life; Dictionary of the English Language; quoted in Clegg, B. 2001. Light Years. Piatkus, London.
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The scientific study of light has more than 1500 years of illustrious history. Beginning with Euclid and his geometrical study of light beams, the list of luminaries includes the great scientist/mathematicians Descartes, Galileo, Snell, Fermat, Boyle, Hooke, Newton, Euler, Fourier, Bartholinus, Huygens, Malus, Gauss, Laplace, Fresnel, Hamilton, Cauchy, Poisson, Faraday, and Maxwell. From those classical beginnings, the theories have evolved into atomic and quantum mechanical theories of light, developed by the great physicists Planck, Bohr, Heisenberg, Schrodinger, Born, Dirac, and Einstein. With such brainpower as driving force, the subsequent profound understanding of light should not have been unexpected.
The first mathematical treatments of light however quickly became mired in an ineluctable ĂŠther; that is, the early physical theory of action at a distance required the presence of an all-pervasive, elastic, and very subtle material to serve as the medium through which forces could transfer their effect. Simply put, although often not easy to apply, the interaction between two separate bodies is determined by a mechanical transfer of force acting along a line connecting the bodies, that force weakening with the distance between the bodies. The action at a distance theory could successfully describe many observations in common experience, the most cogent example being sea waves. But this âĂŠtherealâ view of Nature confounded even its proponents when faced with the equally naturally observed electromagnetic phenomena, such as the effects of a magnet on a current-carrying wire and the invisible transfer of electromagnetic forces through a vacuum.
The great mathematical physicist Maxwell too was caught up in the ĂŠthereal action at a distance and a physics based on mechanics and fluid dynamics, so his initial efforts to mathematically describe the observed electromagnetic phenomena were based on such conceptualizations of electrical energy as the stored energy in a spring, and magnetic energy as the kinetic energy of a flywheel, and of course, electric current was seen as flowing water (an analogy nonetheless still used today). When Maxwell faced the interaction between electricity and magnetism, however, he was confounded: how would an electric current in a wire produce a concentric circulating magnetic force, and how would a moving magnet near a wire coil produce an electric current in that coil? The description of all the parts and the mutual interactions among them using purely mechanistic and fluid formulations would result in some strange machines [1].
For example, the deus ex machina sketched in Figure 2.1 consisting of an interconnected contraption of balls, wheels, gears, and ...
