It is known, these days, that electromagnetic radiation exists at just about every conceivable wavelength. The velocity of this radiation, in empty space, has a fixed value, roughly 3 × 108 m sec-1, regardless of the wavelength. The longer wavelengths are generated by electrical circuits and are used for radio. The very short wavelengths are generated by bombarding metal targets with beams of electrons, the electrons themselves having very nearly the velocity of light; they are used as X-rays. There is a narrow region in the spectrum, from 0.38 to 0.76 × 10-6 m in wavelength, that holds a particular interest. The reason is in our eyes; that is the range of wavelengths, approximately, to which they are sensitive. Radiations in this range of wavelength, and sometimes a bit beyond it, are called light. The units of length used for these short wavelengths are the micron (for which the abbreviation µ will be used only sparingly, since this symbol is also used for refractive index), the millimicron (mµ), and the ångström unit (Å). One meter equals 106 microns equals 109 mµ equals 1010 Å.
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