Amplitude, the maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position. It is equal to one-half the length of the vibration path. The amplitude of a pendulum is thus one-half the distance that the bob traverses in moving from one side to the other. Waves are generated by vibrating sources, their amplitude being proportional to the amplitude of the source.
For a transverse wave, such as the wave on a plucked string, amplitude is measured by the maximum displacement of any point on the string from its position when the string is at rest. For a longitudinal wave, such as a sound wave, amplitude is measured by the maximum displacement of a particle from its position of equilibrium. When the amplitude of a wave steadily decreases because its energy is being lost, it is said to be damped.
Wavelength, distance between corresponding points of two consecutive waves. “Corresponding points” refers to two points or particles in the same phase—i.e., points that have completed identical fractions of their periodic motion. Usually, in transverse waves (waves with points oscillating at right angles to the direction of their advance), wavelength is measured from crest to crest or from trough to trough; in longitudinal waves (waves with points vibrating in the same direction as their advance), it is measured from compression to compression or from rarefaction to rarefaction. Wavelength is usually denoted by the Greek letter lambda (?); it is equal to the speed (v) of a wave train in a medium divided by its frequency (f): ? = v/f.