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The Electromagnetic Spectrum

We have special names for electromagnetic ($EM$) waves of different wavelengths and frequencies.^14.15We call $EM$ waves with $\lambda \gsim 1$ m "radio waves," which are subdivided into various ranges or "bands" like "short wave" (same thing as high frequency), VHF (very high frequency), UHF (ultra high frequency) and so on.^14.16The dividing line between "radar" and "microwave" bands (for example) is determined by arbitrary convention, if at all, but the rule of thumb is that if the wavelength fits inside a very small appliance it is "microwave." Somewhere towards the short end of the microwave spectrum is the beginning of "far infrared," which of course becomes "near infrared" as the wavelength gets still shorter. The name "infrared" is meant to suggest frequencies below those of the red end of the visible light spectrum of $EM$ waves, which extends (depending on the individual eye) from a wavelength of roughly 500 nm (5000 Å) for red light through orange, yellow, green and blue to roughly 200 nm (2000 Å) for violet light. Beyond that we lost sight of the shorter wavelengths (so to speak) and the next range is called "near ultraviolet," the etymology of which is obvious. Next comes "far ultraviolet" which fades into "soft x-rays" and in turn "hard x-rays" and finally "gamma rays" as the frequency increases and the wavelength gets shorter. Note all the different kinds of "rays" that are all just other forms of light - i.e. $EM$ waves - with different wavelengths!

Figure: The electromagnetic spectrum. Note logarithmic wavelength and frequency scales.

Footnotes

. . . frequencies.^14.15

If the wavelength $\lambda$ increases (so that the wavenumber $k = 2\pi/\lambda$ decreases), then the frequency $\omega$ must decrease to match, since the ratio $\omega/k$ must always be equal to the same propagation velocity $c$.

. . . on.^14.16

One can detect a history of proponents of different bands claiming ever higher (and therefore presumably "better") frequency ranges . . . .