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PHYSICAL AND MOLECULAR INTERACTIONS 41

Crossed
light
polarizers

Light transmitted Lower polarizer blocks
the transmission
of light
No voltage applied Voltage applied

Figure 2.2 The transparent electrodes in an LCD are coated with crossed polarizers. The liquid
crystals (depicted as slender lozenges) form helices, thereby ‘guiding’ polarized light from the upper
electrode through the LCD, enabling transmission through to the lower polarizer. This is why the
display has no colour. The helical structure is destroyed when a voltage is applied, because the polar
liquid crystals align with the electrodes’ field. No light can transmit, so the display looks black

that depend on the direction of measurement, because of the alignment of their long,
rod-like structures.
In a liquid-crystal display (LCD) device, the two electrodes
are parallel and separated by a thin layer of liquid crystal (see A physicist would say
Figure 2.2). The liquid crystals in this layer naturally adopt a heli- the liquid crystal
cal structure. adopted a twisted
Light can be represented as a transverse electromagnetic wave nematic structure.
made up of fluctuating electric and magnetic fields, moving in
mutually perpendicular directions (see Chapter 9). Ordinary light is made up of waves
that fluctuate at all possible angles, which normally cannot be separated. A polarizer
is a material that allows only light with a specific angle of vibration to transmit.
We place a light polarizer on one side of either transparent electrode in the LCD,
each similar to one lens in a pair of polaroid sunglasses. The helix of the liquid
crystal twists the polarized light as it transmits through the LCD, guiding it from the
upper polarizer and allowing it unhindered passage through the ‘sandwich’ and lower
polarizer. The transmitting state of an LCD (at zero voltage) is thus ‘clear’.
Applying a voltage to a pixel within the cell causes the mole-
cules to move, aligning themselves parallel with the electric field ‘Pixel’ is short for ‘pic-
imparted by the electrodes. This realignment destroys the helical ture element’. An LCD
structure, precluding the unhindered transmission of light, and the image comprises many
display appears black. thousands of pixels.
Molecules of this type are influenced by an external electric
field because they possess a dipole: one end of the molecule is
electron withdrawing while the other is electron attracting, with the result that one
end possesses a higher electron density than the other. As a result, the molecule
behaves much like a miniature bar magnet. Applying a voltage between the two