megahertz

The megahertz is where radio becomes everyday equipment. FM broadcasting occupies 87.5 to 108 MHz, aircraft talk to towers between 118 and 137 MHz, and the tap of a contactless card happens at 13.56 MHz, a frequency reserved worldwide for short-range tags. Medical ultrasound images at 2 to 18 MHz, trading penetration for detail as the frequency rises. At 100 MHz a radio wave is about 3 m long, from $\lambda = c/f$, the relation the wave solvers on this site work with, and one cycle lasts 10 ns.

Chemists use the megahertz as a name rather than a measurement. An NMR spectrometer is called a "400" or a "600" after the frequency in MHz at which hydrogen nuclei resonate in its magnet, so a 600 MHz machine is really a statement about its 14.1 tesla field, dressed as a frequency. The trap in electronics is bandwidth: a 100 MHz oscilloscope will not display a 100 MHz square wave faithfully, because the harmonics that make its edges square sit at 300 MHz and above, beyond what the instrument passes. And the first IBM PC clocked at 4.77 MHz, which placed personal computing in this band for its first fifteen years before the gigahertz took over.