Q.Define wavelength.

A set of precise terms is needed to compare and distinguish any two wave patterns quantitatively. For a transverse wave, the crest is the highest point of the disturbance above the undisturbed (mean) reference level, and the trough is the lowest point below it; for a longitudinal wave the analogous features are the compression (crowded, high-pressure region) and rarefaction (spread-out, low-pressure region). The wavelength $\lambda$ (SI unit: metre) is the length of one complete, non-repeating section of the wave pattern -- for a transverse wave, the distance between two consecutive crests (or two consecutive troughs); for a longitudinal wave, the distance between two consecutive compressions (or two consecutive rarefactions). The frequency $f$ (SI unit: hertz, Hz) is the number of complete waves that cross a fixed point per second, while the time period $T$ (SI unit: second) is the time taken for exactly one wave to cross that point; because one wave crossing takes $1/f$ seconds, frequency and period are reciprocals of each other, $T = 1/f$. The amplitude $A$ of a wave is the maximum displacement of the medium from its mean (reference) position -- essentially the height of a crest or the depth of a trough measured from the undisturbed level -- and it is amplitude alone, not wavelength, frequency or wave speed, that distinguishes two otherwise-identical sinusoidal waves of different "size". The wave (or phase) velocity $v$ is the distance the wave pattern itself advances in one second, and combining the d …