frequency
Waves need not be restricted to travel through matter. As Maxwell showed, electromagnetic waves consist of an electric field oscillating in step with a perpendicular magnetic field, both of which are perpendicular to the direction of travel. These waves can travel through a vacuum at a constant speed of 2.998 108 m/s, the speed of light denoted by c . All waves, including forms of electromagnetic radiation, are characterized by, a wavelength denoted by , the lowercase Greek letter lambda , a frequency denoted by , the lowercase Greek letter nu , and an amplitude. As can be seen in Figure 6.2, the wavelength is the distance between two consecutive peaks or troughs in a wave measured in meters in the SI system . Electromagnetic waves have wavelengths that fall within an enormous range-wavelengths of kilometers 103 m to picometers 1012 m have been observed. The frequency is the number of wave cycles that pass a specified point in space in a specified amount of time in the SI system, this is measured in seconds . A cycle corresponds to one complete wavelength. The unit for frequency, expressed as cycles per second [s1], is the hertz Hz . Common multiples of this unit are megahertz, 1 MHz = 1 106 Hz and gigahertz 1 GHz = 1 109 Hz . The amplitude corresponds to the magnitude of the wave's displacement and so, in Figure 6.2, this corresponds to one-half the height between the peaks and troughs. The amplitude is related to the intensity of the wave, which for light is the brightness, and for sound is the loudness.