Describing Electromagnetic Waves

Electromagnetic waves can be characterized in terms of their velocity c, the frequency of oscillation n, and the wavelength l. These quantities are related:

2. .


Wavelength is the distance between successive maxima of field strength and has dimensions of length, while frequency has dimensions of inverse time, so velocity is measured in distance per time. In a vacuum the speed of light c=m s^-1. This value can change in other materials depending on the index of refraction m, which varies with frequency. In air  so c is nearly unchanged; in water at visible wavelengths , so within cloud drops the speed of light is diminished by about 25%. The inverse of wavelength is the wavenumber . In atmospheric applications wavelength is commonly measured in microns (1µm=10^-6m), nanometers (1 nm=10^-9 m), or Angstroms (1Å=10^-10m), with frequency in megahertz (1MHz=10⁶ s^-1= 0⁶ Hz) or gigahertz (1 GHz=10⁹ Hz) and wavenumber expressed in inverse centimeters.

Name	Spectral Region
X-rays	l < 10nm
Ultraviolet (UV)	10 < l < 400nm
Visible	0.4 < l < 0.7µm
Near-Infrared (Near-IR)	0.7 < l < 3.5µm
Middle-IR	3.5< l < 30µm
Far-IR	30 < l < 100µm
Microwave	1mm<l<1m

The plane in which the electric field oscillates determines the polarization of the radiation. In the atmosphere, though, this plane is rarely constant (i.e. radiation in the atmosphere is usually unpolarized) so we’ll ignore this aspect.

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