e preceding formula the speed of light is approximately 3 X 108 meters/second. We can also compute wavelength based on frequencies in kHz, MHz, and GHz by varying the numerator and denominator in the prior equation as follows: Appendix A v(m)=3 x 10'/f(kHz) w (m) = 300/f(MHz) w(m)=0.3/f(GHz) In the beginning of this appendix we noted that we could refer to a station on a radio dial in one of three ways. If we use the previously noted relationships between frequency, wavelength, and velocity of light we can now note that a frequency of 4250 kHz would be equivalent to a wavelength of 70.72 meters, which we would round to 71 meters. Thus, the operating terms of 4250 kHz, 4.25 MHz, and 71 meters all refer to the same operating frequency! As the preceding use of the relationships between frequency, wave- length, and velocity of light indicates, the wavelength of a signal decreases as its frequency increases. Although today frequencies are not commonly given in terms of wavelength in the world of radio communications, in the world of optical communications the two are used interchangeably. The electromagnetic spectrum is a continuum of wavelengths, from zero to infinity. Physicists name regions of the spectrum based on the type of wavelength in the region. Some of the named regions include: gamma rays, which have the shortest wavelengths (<0.01 nm, where nm stands for nanometer and represents a billionth of a meter) and high- est frequency; X-rays that range in wavelength from 0.01-10 nm; ultra- violet radiation, which has wavelengths of 10-310 nm; visible light, which covers the range of wavelengths from 400-700 nm; and infrared, which has wavelengths spanning 710 nm-1 mm. In optical communi- cations we are primarily interested in visible light and infrared light. The wavelength of light is SO small that it is conveniently expressed in nanometers (1x10-9) or millimicrons (mu), which are equal to one- billionth of a meter or one-thousandth of a micron. The wavelength of violet light varies from about 400 to 450 mm, and