Connection for AP® Courses

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Connection for AP® Courses
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OpenStax
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Figure 1:
Human eyes detect these orange sea goldie sh swimming over a coral reef in the blue waters
of the Gulf of Eilat (Red Sea) using visible light. (credit: Daviddarom, Wikimedia Commons)
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 popcornall involve
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electromagnetic waves.
The
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list of the various types of electromagnetic waves, ranging from radio transmission waves to nuclear gammarays (γ -rays), is interesting in itself. Even more intriguing is that all of these widely varied phenomena are
dierent manifestations of the same thingelectromagnetic waves. (See Figure 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 eects?
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 elds, 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 dene 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 dierent types of electromagnetic radiation that are characterized by their wavelengths (Essential Knowledge 6.F.1) and have been
given specic names (see Figure 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 dierent 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 specic 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.
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Essential Knowledge 6.F.2 Electromagnetic waves can transmit energy through a medium and through
a vacuum.
:
Many people confuse sound waves with
radio waves,
one type of electromagnetic (EM) wave.
However, sound and radio waves are completely dierent 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.
1 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 eects 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 verication 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.
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Figure 2:
The electromagnetic waves sent and received by this 50-foot radar dish antenna at Kennedy
Space Center in Florida are not visible, but help track expendable launch vehicles with high-denition
imagery. The rst use of this C-band radar dish was for the launch of the Atlas V rocket sending the
New Horizons probe toward Pluto. (credit: NASA)
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