Introduction

In the previous chapter we saw that, when electric dipoles are forced to oscillate, they induce an electric field that oscillates at the same frequency. In addition, owing to the motion of the oscillating charges, a magnetic field oscillating at the same frequency is also induced. These simultaneous oscillating fields are the basis for all known modes of electromagnetic radiation. Thus, Xrays, UV radiation, visible light, and infrared and microwave radiation are all part of the same physical phenomenon. Although each radiating mode is significantly different from the others, all modes of electromagnetic radiation can be described by the same equations because they all obey the same basic laws.

Oscillation alone is insufficient to account for electromagnetic radiation. The other important observation is that radiation propagates. It is broadcast by a source and, if uninterrupted, can propagate indefinitely in both time and space. An example of the boundless propagation of electromagnetic waves – whether X-ray, visible, or microwave – is the radiation emitted by remote galaxies. Some of this radiation, generated at primordial times and at remote reaches of the universe, can be detected on earth billions of years later. Evidently, radiation is not limited to the immediate vicinity of the source. Although we know that certain media can block radiation, we find it more astonishing that electromagnetic waves can propagate through free space; unlike electrical currents or sound, conductors are not necessary for the transmission of radiation.