An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

7.3 Polarization of electromagnetic radiation

So far we have described electromagnetic radiation in terms of its wavelength, frequency and speed. It has another, sometimes important, property: polarization. Figure 10 shows the electric and magnetic field in a plane-polarized electromagnetic wave. In any electromagnetic radiation, the electric and magnetic fields are always perpendicular to each other, as well as perpendicular to the direction of propagation of the wave. In plane-polarized radiation the electric field vector always lies in a single plane, the vertical plane in the example shown in Figure 10.

Figure 10
Figure 10 Diagram illustrating the relative orientations of the electric and magnetic field vectors in plane-polarized light

In general, sources of electromagnetic radiation are not polarized. In such unpolarized light, each photon has its own, randomly oriented, electric field direction. Only if all the photons in the radiation are oriented somehow, will net polarization occur. There are several ways this can happen. The most familiar everyday example is the plane polarization which occurs because of reflection. When a light ray is reflected the electric field vector of the incident ray will, in general, have two non-zero components as shown in Figure 11: one perpendicular to the reflecting surface, i.e. parallel to the page (represented with arrows in Figure 11), and one parallel to the reflecting surface, i.e. perpendicular to the page (dots in Figure 11 represent the arrow tips of this component). These two components are reflected with different efficiencies, so reflected light is consequently partially or totally plane-polarized. This is why Polaroid sunglasses are so effective at cutting down the glare from sunlight reflected by water or glass. In astrophysics, light is often scattered by dust, and this scattered light becomes polarized in the same way. Synchrotron radiation, the subject of the next section, is intrinsically polarized: it is emitted as a result of electrons interacting with a magnetic field, and the orientation of the polarization of synchrotron radiation is governed by the orientation of the magnetic field.

Note: you may have previously studied electromagnetism in SI units. In this case you may have seen the Poynting vector expressed as S = × H, which is equivalent to the cgs version given here.

The flux of energy associated with an electromagnetic wave is given by the Poynting vector, where here and B are the fields comprising the electromagnetic wave. Referring to Figure 10, and applying the rule for forming a vector product you can see that the Poynting vector is clearly along the direction of propagation of the wave.

Figure 11
Figure 11 Reflection of light at a surface. The arrows indicate the component of the electric field vector which is in the plane of the page, the dots represent the electric field component which is parallel to the reflecting surface. If the angle of incidence is equal to the Brewster angle, θB, the reflected ray is 100 per cent polarized, as shown. For other angles of incidence the reflected ray is only partially polarized

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