Polarization is a characteristic of all transverse waves. This chapter is about light, but to introduce some basic polarization concepts, let’s go back to the transverse waves on a string that we studied in Chapter 19. For a string that in equilibrium lies along the z-axis, the displacements may be along the y-direction, as in Fig. 34-I(>a. In this case the string always lies in the xy-plane. But the displacements might instead be along the z-axis, as in Fig. 34-16b; then the string always lies in the xz-plane,When a wave has only y-displacements, we say that it is linearly polarized in the y-direction; a wave with only z-displacements is linearly polarized in the z-direction.
For mechanical waves we can build a polarizing filter, or polarizer, that permits only waves with a certain polarization direction to pass. In Fig. 34-16c the string can slide vertically in the slot without friction, but no horizontal motion is possible. This filter passes waves that are polarized in the y-direction but blocks those that are polarized in the z-direction. This same language can be applied to electromagnetic waves, which also have polarization. As we learned in Chapter 33, an electromagnetic wave is a transverse wave; the fluctuating electric and magnetic fields are perpendicular to each other and to the direction of propagation. We always define the direction ofgolarization of an electromagnetic wave to be the direction of the electric-field vector E, not the magnetic field, because many common electromagnetic-wave detectors respond to the electric forces on electrons in materials, not the magnetic forces. Thus the electromagnetic wave described by Eq. (33-17),
CAUTION~ It’s unfortunate that the same word “polarization” that is used to describe the direction of E in an electromagnetic wave is also used to describe the shifting of electric charge within a body, such as in response to a nearby charged body; we described this latter kind of polarization in Section 22-4 (see Fig. 22-5). You should remember that while these two concepts have the same name, they are not the same .•