31.7 Polarization
Liquid crystal displays (LCDs) are ubiquitous—from your digital watch to your phone to your TV. Liquid crystals have rod-shaped molecules that can flow like a liquid while staying aligned with one another like the atoms in a crystal. The aligned molecules in each pixel of an LCD act as a polarizing filter whose axis can be changed or rotated with an applied voltage. Bright polarized light from the back of the display passes through the liquid crystal array and is then, depending on each pixel’s voltage, fully, partially, or not blocked by a crossed polarizer (see Figure 31.30) at the front of the display. Hundreds, thousands, or even millions of pixels of variable intensity produce the image you see on the display.
The plane of the electric field vector and the Poynting vector (the direction of propagation) is called the plane of polarization of an electromagnetic wave. FIGURE 31.27 shows two electromagnetic waves moving along the x-axis. The electric field in Figure 31.27a oscillates vertically, so we would say that this wave is vertically polarized. Similarly the wave in Figure 31.27b is horizontally polarized. Other polarizations are possible, such as a wave polarized away from horizontal.
FIGURE 31.27 The plane of polarization is the plane in which the electric field vector oscillates.
NOTE This use of the term “polarization” is completely independent of the idea of charge polarization that you learned about in Chapter 22.
Some wave sources, such as lasers and radio antennas, emit polarized electromagnetic waves with a well-defined plane of polarization. By contrast, most natural sources of electromagnetic radiation are unpolarized, emitting waves whose electric fields oscillate randomly with all possible orientations.
A few natural sources are partially polarized, meaning that one direction of polarization is more prominent than others. The light of the sky at right angles to the sun is partially polarized because of how the sun’s light scatters from air molecules to create skylight. Bees and other insects make use of this partial polarization to navigate. Light reflected from a flat, horizontal surface, such as a road or the surface of a lake, has a predominantly horizontal polarization. This is the rationale for using polarizing sunglasses.
The most common way of artificially generating polarized visible light is to send unpolarized light through a polarizing filter. The first widely used polarizing filter was invented by Edwin Land in 1928, while he was still an undergraduate student. He developed an improved version, called Polaroid, in 1938. Polaroid, as shown in FIGURE 31.28, is a plastic sheet containing very long organic molecules known as polymers. The sheets are formed in such a way that the polymers are all aligned to form a grid, rather like the metal bars in a barbecue grill. The sheet is then chemically treated to make the polymer molecules somewhat conducting.
FIGURE 31.28 A polarizing filter.
As a light wave travels through Polaroid, the component of the electric field oscillating parallel to the polymer grid drives the conduction electrons up and down the molecules. The electrons absorb energy from the light wave, so the parallel component of is absorbed in the filter. But the conduction electrons can’t oscillate perpendicular to the molecules, so the component of perpendicular to the polymer grid passes through without absorption. Thus the light wave emerging from a polarizing filter is polarized perpendicular to the polymer grid. The direction of the transmitted polarization is called the polarizer axis.