2.2 Waves in What?

Waves of radiation differ in one fundamental respect from water waves, sound waves, or any other waves that travel through a material medium—radiation needs no such medium. When light travels from a distant cosmic object, it moves through the virtual vacuum of space. Sound waves, by contrast, cannot do this, despite what you have probably heard in almost every sci-fi movie ever made! If we were to remove all the air from a room, conversation would be impossible (even with suitable breathing apparatus to keep our test subjects alive) because sound waves cannot exist without air or some other physical medium to support them. Communication by flashlight or radio, however, would be entirely feasible.

The ability of light to travel through empty space was once a great mystery. The idea that light, or any other kind of radiation, could move as a wave through nothing at all seemed to violate common sense, yet it is now a cornerstone of modern physics.

Interactions Between Charged Particles

Figure 2.4 Charged Particles (a) Particles carrying like electrical charges repel one another; particles with unlike charges attract. (b) A charged particle is surrounded by an electric field, which determines the particle’s influence on other charged particles. We represent the field by a series of field lines. (c) If a charged particle begins to vibrate, its electric field changes. The resulting disturbance travels through space as a wave.

To understand more about the nature of light, consider anelectrically charged particle, such as an electron or aproton. Electrons and protons are elementary particles—fundamental components of matter—that carry the basic unit of charge. Electrons are said to carry a negative charge, while protons carry an equal and opposite positive charge. Just as a massive object exerts a gravitational force on any other massive object, an electrically charged particle exerts an electrical force on every other charged particle in the universe. (Sec. 1.4) Unlike gravity, however, which is always attractive, electrical forces can be either attractive or repulsive. Particles having like charges (both negative or both positive) repel one another; particles having unlike charges attract (Figure 2.4a).Extending outward in all directions from our charged particle is an electric field, which determines the electric force exerted by the particle on other charged particles (Figure 2.4b). The strength of the electric field, like that of the gravitational field, decreases with increasing distance from the source according to an inverse-square law. By means of the electric field, the particle’s presence is “felt” by other charged particles, near and far.

Now suppose our particle begins to vibrate, perhaps because it becomes heated or collides with some other particle. Its changing position causes its associated electric field to change, and this changing field in turn causes the electrical force exerted on other charges to vary (Figure 2.4c). If we measure the changes in the forces on these other charges, we learn about our original particle. Thus, information about our particle’s motion is transmitted through space via a changing electric field. This disturbance in the particle’s electric field travels through space as a wave.

Electromagnetic Waves

	Figure 2.5 Magnetism Earth’s magnetic field interacts with a magnetic compass needle, causing the needle to become aligned with the field—that is, to point toward Earth’s north (magnetic) pole.

The laws of physics tell us that a magnetic field must accompany every changing electric field. Magnetic fields govern the influence of magnetized objects on one another, much as electric fields govern interactions between charged particles. The fact that a compass needle always points to magnetic north is the result of the interaction between the magnetized needle and Earth’s magnetic field (Figure 2.5). Magnetic fields also exert forces on moving electric charges (that is, electric currents)—electric meters and motors rely on this basic fact. Conversely, moving charges create magnetic fields (electromagnets are a familiar example).

Electric and magnetic fields are inextricably linked to one another. A change in either onenecessarily creates the other. For this reason, the disturbance produced by our moving charge actually consists of oscillating electric and magnetic fields, always oriented perpendicular to one another and moving together through space (Figure 2.6). These fields do not exist as independent entities. Rather, they are different aspects of a single physical phenomenon: electromagnetism. Together they constitute an electromagnetic wave that carries energy and information from one part of the universe to another.

Electromagnetic Waves

Now consider a distant cosmic object—a star. It is made up of charged particles, mainly protons and electrons, in constant motion. As these charged contents move around, their electric fields change, and electromagnetic waves are produced. These waves travel outward into space, and eventually some reach Earth. Other charged particles, either in our eyes or in our experimental apparatus, respond to the electromagnetic field changes by vibrating in tune with the received radiation. This response is how we “see” the radiation—with our eyes or with our detectors.

Figure 2.6 Electromagnetic Wave Electric and magnetic fields vibrate perpendicular to each other. Together they form an electromagnetic wave that moves through space at the speed of light.

All electromagnetic waves move at a very specific speed—the speed of light (always denoted by the letter c). Its value is 299,792.458 km/s in a vacuum (and somewhat less in material substances, such as air or water). In this text, we round this value off to  This is an extremely high speed. In the time needed to snap a finger—about a tenth of a second—light can travel three quarters of the way around our planet! According to the Theory of Relativity (see More Precisely 13-1), the speed of light is the fastest speed possible.

CONCEPT CHECK

What is light? List some similarities and differences between light waves and waves on water or in air.