Key Terms

displacement current

extra term in Maxwell’s equations that is analogous to a real current but accounts for a changing electric field producing a magnetic field, even when the real current is present

gamma ray ($\gamma$ ray)

extremely high frequency electromagnetic radiation emitted by the nucleus of an atom, either from natural nuclear decay or induced nuclear processes in nuclear reactors and weapons; the lower end of the $\gamma$ -ray frequency range overlaps the upper end of the X-ray range, but $\gamma$ rays can have the highest frequency of any electromagnetic radiation

infrared radiation

region of the electromagnetic spectrum with a frequency range that extends from just below the red region of the visible light spectrum up to the microwave region, or from $0.74\mu \text{m to}300\mu \text{m}$

Maxwell’s equations

set of four equations that comprise a complete, overarching theory of electromagnetism

microwaves

electromagnetic waves with wavelengths in the range from 1 mm to 1 m; they can be produced by currents in macroscopic circuits and devices

Poynting vector

vector equal to the cross product of the electric-and magnetic fields, that describes the flow of electromagnetic energy through a surface

radar

common application of microwaves; radar can determine the distance to objects as diverse as clouds and aircraft, as well as determine the speed of a car or the intensity of a rainstorm

radiation pressure

force divided by area applied by an electromagnetic wave on a surface

radio waves

electromagnetic waves with wavelengths in the range from 1 mm to 100 km; they are produced by currents in wires and circuits and by astronomical phenomena

thermal agitation

thermal motion of atoms and molecules in any object at a temperature above absolute zero, which causes them to emit and absorb radiation

ultraviolet radiation

electromagnetic radiation in the range extending upward in frequency from violet light and overlapping with the lowest X-ray frequencies, with wavelengths from 400 nm down to about 10 nm

visible light

narrow segment of the electromagnetic spectrum to which the normal human eye responds, from about 400 to 750 nm

X-ray

invisible, penetrating form of very high frequency electromagnetic radiation, overlapping both the ultraviolet range and the $\gamma$-ray range