WAVE VELOCITY

distance traversed by a periodic, or cyclic, motion per unit time (in any direction). Wave velocity in common usage refers to speed, although, properly, velocity implies both speed and direction. The velocity of a wave is equal to the product of its wavelength and frequency (number of vibrations per second) and is independent of its intensity. If a point vibrates within a rigid solid, both transverse waves (those with points oscillating at right angles to the direction of their advance) and longitudinal waves (those with points vibrating the same direction as their advance) of the same frequency are sent out, and, because the longitudinal waves happen to have longer wavelengths, they will move faster. Thus, seismic waves, being composed of both longitudinal waves (P, primary) and transverse waves (S, secondary), move with two velocities through the Earth. Longitudinal waves, such as sound, are transmitted through media with velocities depending on the density and elasticity of the substance. Sound has a velocity of about 0.33 km per second (0.2 mile per second) in air, 1.5 km per second in water, and 5 km per second in steel. All sound waves travel with the same speed in air regardless of their frequency. The velocity of light in vacuo is also independent of frequency; in a transparent medium, however, the velocity of light depends on the effect of dispersion in the material, varying slightly more than 1 percent from blue to red.