24: Electromagnetic Waves
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.
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- 24.0: Prelude to Electromagnetic Waves
- Even more intriguing is that all of these widely varied phenomena are different manifestations of the same thing—electromagnetic waves. (See Figure 2.) What are electromagnetic waves? How are they created, and how do they travel? How can we understand and organize their widely varying properties? What is their relationship to electric and magnetic effects? These and other questions will be explored.
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- 24.1: Maxwell’s Equations- Electromagnetic Waves Predicted and Observed
- Electromagnetic waves consist of oscillating electric and magnetic fields and propagate at the speed of light \(c\). They were predicted by Maxwell, who also showed that \[c = \frac{1}{\sqrt{\mu_{0} \epsilon_{0}}},\] where \(mu_{0}\) is the permeability of free space and \(\epsilon_{0}\) is the permitivity of free space. Maxwell’s prediction of electromagnetic waves resulted from his formulation of a complete and symmetric theory of electricity and magnetism, known as Maxwell’s equations.
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- 24.2: Production of Electromagnetic Waves
- Electromagnetic waves are created by oscillating charges (which radiate whenever accelerated) and have the same frequency as the oscillation. Since the electric and magnetic fields in most electromagnetic waves are perpendicular to the direction in which the wave moves, it is ordinarily a transverse wave. The strengths of the electric and magnetic parts of the wave are related by \[\frac{E}{B} = c,\] which implies that the magnetic field \(B\) is very weak relative to the electric field \(E\).
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- 24.3: The Electromagnetic Spectrum
- In this module we examine how electromagnetic waves are classified into categories such as radio, infrared, ultraviolet, and so on, so that we can understand some of their similarities as well as some of their differences. We will also find that there are many connections with previously discussed topics, such as wavelength and resonance.
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- 24.4: Energy in Electromagnetic Waves
- The energy carried by any wave is proportional to its amplitude squared. For electromagnetic waves, this means intensity can be expressed as \[I_{ave} = \frac{c \epsilon_{0} E_{0}^{2}}{2},\] where \(I_{ave}\) is the average intensity in \(W/m^{2}\), and \(E_{0}\) is the maximum electric field strength of a continuous sinusoidal wave. This can also be expressed in terms of the maximum magnetic field strength and in terms of both electric and magnetic fields.