Although you may not know what the terms stand for, you have probably heard of shortwave radio, microwaves, and VHF and UHF television signals. Each of these terms is used to describe one of the many types of electromagnetic waves that are broadcast through the air to provide you with radio, television, and other forms of communication. All of these waves consist of electric and magnetic fields propagating through space.

The key to understanding how these waves behave is understanding the nature of the electron. Why? Because electromagnetic waves are produced by accelerating electrons, the electrons charge produces electric fields and the electrons motion produces magnetic fields. Furthermore, the waves are broadcast and received by antennas, devices made of matter that also contains electrons. Thus, the logical first step in understanding how electromagnetic waves are produced, propagated, received, and used for so many devices is to learn about the properties of the electron.

We saw that energy is transported to us from the sun via a class of waves known as electromagnetic waves. This class includes the familiar visible, ultraviolet, and infrared waves. And we studied the concepts of electric and magnetic fields. It was the great Scottish physicist James Clerk Maxwell (1831-1879) who showed that these two fields fluctuating together can form a propagating electromagnetic wave. We will now bring together our knowledge of electric and magnetic fields in order to understand this important type of wave.

Production of Electromagnetic Waves

Lets illustrates one way to create an electromagnetic wave. The setup consists of two straight metal wires that are connected to the terminals of an ac generator and serve as an antenna. The potential difference between the terminals changes sinusoidally with time $t$ and has a period $T$. At the instant $t = 0$ s, when there is no charge at the ends of either wire. Since there is no charge, there is no electric field at the point P just to the right of the antenna. As time passes, the top wire becomes positively charged and the bottom wire negatively charged. One-quarter of a cycle later ( $t = 1/4 T$), the charges have attained their maximum values. The corresponding electric field $\vec{E}$ at point P has increased to its maximum strength in the downward direction. Also the electric field created at earlier times has not disappeared but has moved to the right. Here lies the crux of the matter: At distant points, the electric field of the charges is not felt immediately. Instead, the field is created first near the wires and then, like the effect of a pebble dropped into a pond, moves outward as a wave in all directions.

Along with the electric field, a magnetic field $\vec{B}$ is also created, because the charges flowing in the antenna constitute an electric current, which produces a magnetic field. With the aid of RHR2 (thumb of the right hand points along the current $I$, fingers curl in the direction of $\vec{B}$), the magnetic field at P can be seen to point into the page. As the oscillating current changes, the magnetic field changes accordingly. The magnetic fields created at earlier times propagate outward as a wave, just as the electric fields do.

Notice that the magnetic field perpendicular to the page, whereas the electric field lies in the plane of the page. Thus, the electric and magnetic fields created by the antenna are mutually perpendicular and remain so as they move outward. Moreover, both fields are perpendicular to the direction of travel. These perpendicular electric and magnetic fields, moving together, constitute an electromagnetic wave.

An electromagnetic wave, like any periodic wave, has frequency $f$ and wavelength $\lambda$ that are related to the speed $v$ of the wave by $v = f \lambda$. For electromagnetic waves traveling through a vacuum or, to a good approximation, through air, the speed is $v = c$, so $c= f \lambda$.

Electromagnetic waves exist with an enormous range of frequencies, from values less than $10^4$ Hz to greater than $10^{24}$ Hz. Since all these waves travel through a vacuum at the same speed of $c = 3.00 \times 10^8$ m/s, the above Equation can be used to find the correspondingly wide range of wavelengths. The ordered series of electromagnetic wave frequencies or wavelengths is called the electromagnetic spectrum. Historically, regions of the spectrum have been given names such as radio waves and infrared waves. Although the boundary between adjacent regions is shown as a sharp line in any drawing, the boundary is not so well defined in practice, and the regions often overlap.

Electromagnetic waves, like water waves or sound waves, carry energy. The energy is carried by the electric and magnetic fields that comprise the wave. In a microwave oven, for example, microwaves penetrate food and deliver their energy to it.

As we know about Doppler effect, it is the effect that sound waves exhibit when either the source of a sound wave, the observer of the wave, or both are moving with respect to the medium of propagation (e.g., air). This effect is one in which the observed sound frequency is greater or smaller than the frequency emitted by the source. A different Doppler effect arises when the source moves than when the observer moves.

Electromagnetic waves also can exhibit a Doppler effect, but it differs from that for sound waves for two reasons. First, sound waves require a medium such as air in which to propagate. In the Doppler effect for sound, it is the motion ( of the source, the observer, and the waves themselves) relative to this medium that is important. In the Doppler effect for electromagnetic waves, motion relative to a medium plays no role, because the waves do not require a medium in which to propagate. They can travel in a vacuum. Second, in the equations for the Doppler effect, the speed of sound plays an important role, and it depends on the reference frame relative to which it is measured. For example, the speed of sound with respect to moving air is different than it is with respect to stationary air. As we will see later in relativity theory, electromagnetic waves behave in a different way. The speed at which they travel has the same value, whether it is measured relative to a stationary observer or relative to one moving at a constant velocity. For these two reasons, the same Doppler effect arises for electromagnetic waves when either the source or the observer of the waves moves; only the relative motion of the source and the observer with respect to one another is important.

One of the essential features of electromagnetic waves is that they are transverse waves, and because of this feature they can be polarized. The idea of polarization can be shown by a transverse wave as it travels along a rope toward a slit. The wave is said to be linearly polarized, which means that its vibrations always occur along one direction. This direction is called the direction of polarization. The direction of polarization is vertical, parallel to the slit Consequently, the wave passes through easily. However, when the slit is turned perpendicular to the direction of polarization, the wave cannot pass, because the slit prevents the rope from oscillating. For longitudinal waves, such as sound waves, the notion of polarization has no meaning. In a longitudinal wave the direction of vibration is along the direction of travel, and the orientation of the slit would have no effect on the wave.