Electromagnetic waves are all around us. The beauty of a coral reef, the warmth of sunshine, sunburn, an X-ray image revealing a broken bone, even microwave popcorn—all involve electromagnetic waves. The list of the various types of electromagnetic waves, ranging from radio transmission waves to nuclear gamma-rays (γ-rays), is interesting in itself. Even more intriguing is that all of these widely varied phenomena are different manifestations of the same thing—electromagnetic waves. (See Figure 24.2.)
What are electromagnetic waves? How are they created, and how do they travel? How can we understand and conceptualize their widely varying properties? What is their relationship to electric and magnetic effects? These and other questions will be explored in this chapter.
Electromagnetic waves support Big Idea 6 that waves can transport energy and momentum. In general, electromagnetic waves behave like any other wave, as they are traveling disturbances (Enduring Understanding 6.A). They consist of oscillating electric and magnetic fields, which can be conceived of as transverse waves (Essential Knowledge 6.A.1). They are periodic and can be described by their amplitude, frequency, wavelength, speed, and energy (Enduring Understanding 6.B).
Simple waves can be modeled mathematically using sine or cosine functions involving the wavelength, amplitude, and frequency of the wave. (Essential Knowledge 6.B.3). However, electromagnetic waves also have some unique properties compared to other waves. They can travel through both matter and a vacuum (Essential Knowledge 6.F.2), unlike mechanical waves, including sound, that require a medium (Essential Knowledge 6.A.2).
Maxwell’s equations define the relationship between electric permittivity, the magnetic permeability of free space (vacuum), and the speed of light, which is the speed of propagation of all electromagnetic waves in a vacuum. This chapter uses the properties electric permittivity (Essential Knowledge 1.E.4) and magnetic permeability (Essential Knowledge 1.E.5) to support Big Idea 1 that objects and systems have certain properties and may have internal structure.
The particular properties mentioned are the macroscopic results of the atomic and molecular structure of materials (Enduring Understanding 1.E). Electromagnetic radiation can be modeled as a wave or as fundamental particles (Enduring Understanding 6.F). This chapter also introduces different types of electromagnetic radiation that are characterized by their wavelengths (Essential Knowledge 6.F.1) and have been given specific names (see Figure 24.2).
Big Idea 1 Objects and systems have properties such as mass and charge. Systems may have internal structure.
Enduring Understanding 1.E Materials have many macroscopic properties that result from the arrangement and interactions of the atoms and molecules that make up the material.
Essential Knowledge 1.E.4 Matter has a property called electric permittivity.
Essential Knowledge 1.E.5 Matter has a property called magnetic permeability.
Big Idea 6. Waves can transfer energy and momentum from one location to another without the permanent transfer of mass and serve as a mathematical model for the description of other phenomena.
Enduring Understanding 6.A A wave is a traveling disturbance that transfers energy and momentum.
Essential Knowledge 6.A.1 Waves can propagate via different oscillation modes such as transverse and longitudinal.
Essential Knowledge 6.A.2 For propagation, mechanical waves require a medium, while electromagnetic waves do not require a physical medium. Examples include light traveling through a vacuum and sound not traveling through a vacuum.
Enduring Understanding 6.B A periodic wave is one that repeats as a function of both time and position and can be described by its amplitude, frequency, wavelength, speed, and energy.
Essential Knowledge 6.B.3 A simple wave can be described by an equation involving one sine or cosine function involving the wavelength, amplitude, and frequency of the wave.
Enduring Understanding 6.F Electromagnetic radiation can be modeled as waves or as fundamental particles.
Essential Knowledge 6.F.1 Types of electromagnetic radiation are characterized by their wavelengths, and certain ranges of wavelength have been given specific names. These include (in order of increasing wavelength spanning a range from picometers to kilometers) gamma rays, x-rays, ultraviolet, visible light, infrared, microwaves, and radio waves.
Essential Knowledge 6.F.2 Electromagnetic waves can transmit energy through a medium and through a vacuum.
Misconception Alert: Sound Waves vs. Radio Waves
Many people confuse sound waves with radio waves, one type of electromagnetic (EM) wave. However, sound and radio waves are completely different phenomena. Sound creates pressure variations (waves) in matter, such as air or water, or your eardrum. Conversely, radio waves are electromagnetic waves, like visible light, infrared, ultraviolet, X-rays, and gamma rays. EM waves don’t need a medium in which to propagate; they can travel through a vacuum, such as outer space.
A radio works because sound waves played by the D.J. at the radio station are converted into electromagnetic waves, then encoded and transmitted in the radio-frequency range. The radio in your car receives the radio waves, decodes the information, and uses a speaker to change it back into a sound wave, bringing sweet music to your ears.
Discovering a New Phenomenon
It is worth noting at the outset that the general phenomenon of electromagnetic waves was predicted by theory before it was realized that light is a form of electromagnetic wave. The prediction was made by James Clerk Maxwell in the mid-19th century when he formulated a single theory combining all the electric and magnetic effects known by scientists at that time. “Electromagnetic waves” was the name he gave to the phenomena his theory predicted.
Such a theoretical prediction followed by experimental verification is an indication of the power of science in general, and physics in particular. The underlying connections and unity of physics allow certain great minds to solve puzzles without having all the pieces. The prediction of electromagnetic waves is one of the most spectacular examples of this power. Certain others, such as the prediction of antimatter, will be discussed in later modules.