[Illustration: _Argon laser, which emits high-power blue-green beam continuously, has application in signal processing, communications, and spectroscopy. This unit is being beamed through prisms that separate its several discrete wavelengths of light, displayed on card at left foreground._] FOOTNOTES [1]Sometimes referred to as _hertz_ (abbreviated Hz), for the 19th Century German physicist Heinrich Hertz; 1000 Hz = 1000 cps. [2]Devised in France and officially adopted there in 1799, the metric system uses the meter as the basic unit of length and has been proposed for all measurements in this country. [3]Named for the Swedish physicist Anders J. Angstrom. [4]The wavelength, indicated by the Greek letter λ (lambda) is related to frequency (f) in the proportion λ (in meters) = 300,000,000/f. (The number 300,000,000 is the velocity of light in meters per second.) [5]Microwaves are radio waves with frequencies above 1000 megacycles per second. [6]Ten to 30,000,000 kilocycles per second; this is low in the electromagnetic spectrum, but not low in terms of the radio spectrum, which has a low-frequency classification of its own. [7]Primitive as early radios were by today’s standards, they brought a new era to communication at the time. Unmodulated CW (continuous wave) transmissions and crystal receivers were used to summon rescuers in the _Titanic_ disaster of 1912, for example. [8]Energy = h (Planck’s constant) × frequency. Planck’s constant is the energy of 1 quantum of radiation, and equals 6.62556 × 10⁻²⁷ erg-sec. [9]Each photon carries 1 _quantum_ of radiation energy, which is a unit equal to the product of the radiation frequency and Planck’s constant (see footnote page 15). [10]Einstein was awarded the Nobel Prize in 1921 for his 1905 explanation of the photoelectric effect (in terms of quanta of energy) and _not_ for his relativity theory. [11]Einstein’s theoretical explanation applies in the case of stimulation of a single atom. In practical stimulation, directionality is enhanced by stimulating many atoms in phase. [12]An atomic clock is a device that uses the extremely fast vibrations of molecules or atomic nuclei to measure time. These vibrations remain constant with time, consequently short intervals can be measured with much higher precision than by mechanical or electrical clocks. [13]The 1966 Nobel Prize in Physics was awarded to Prof. Alfred Kastler of the University of Paris for his research on optical pumping and studies on the energy levels of atoms. [14]See _Accelerators_, a companion booklet in this series, for a full account of the Stanford “Atom Smasher”. [15]For descriptions of fission and fusion processes, see _Controlled Nuclear Fusion_, _Nuclear Reactors_, and _Nuclear Power Plants_, other booklets in this series.