Physics · Waves · NEET
No, but they are the matching parts of two different wave types. A crest belongs to a transverse wave (like a wave on a string) and is the point of maximum positive displacement. A compression belongs to a longitudinal wave (like sound) and is the region of maximum density and pressure. Both mark the peak of the disturbance, so a crest is analogous to a compression, and a trough is analogous to a rarefaction. But you should never call a sound peak a 'crest' in a NEET answer.
They are opposite extremes of the same transverse wave. A crest is where a particle of the medium is displaced the maximum amount in the positive direction (say, upward). A trough is where the particle is displaced the maximum amount in the negative direction (downward). The vertical distance from the rest line to a crest equals the amplitude, and the same is true for a trough. The straight-line distance from one crest to the next crest is one wavelength.
Only in longitudinal waves, such as sound in air. In a longitudinal wave the particles move back and forth along the same direction the wave travels. When particles bunch up, that dense zone is a compression; when they spread out, that thin zone is a rarefaction. Transverse waves do not have compressions or rarefactions; they have crests and troughs instead.
The compression. In a compression the air molecules are packed close together, so both the density and the pressure are maximum there. In a rarefaction the molecules are spread apart, so the density and pressure are minimum. This is why sound is often described as a pressure wave, and the pressure variation is largest at the centres of compressions and rarefactions.
The distance between two successive crests (or two successive troughs) equals one wavelength. In the same way, the distance between two successive compressions (or two successive rarefactions) also equals one wavelength. This is a common NEET link: whether the wave is transverse or longitudinal, wavelength is the distance between two nearest points that are in the same phase.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Sound in air is a longitudinal wave, so it travels as compressions and rarefactions, not crests and troughs. The air particles vibrate along the same line in which the sound moves.
No. A pure transverse wave has crests and troughs, while a pure longitudinal wave has compressions and rarefactions. A given simple wave is one type or the other. Surface water waves are a special mixed case, but for NEET treat the two categories separately.
Amplitude is the maximum displacement of a particle from its rest position, which is exactly the height of a crest above the rest line (or the depth of a trough below it). For sound, amplitude corresponds to the maximum change in density or pressure at a compression.
At a crest or a trough the particle is momentarily at rest (velocity zero) because it is at its extreme position. The particle velocity is maximum as the particle passes through the mean (rest) position, halfway between a crest and a trough.
No. The distance between a crest and the very next trough is half a wavelength. One full wavelength is the distance between two successive crests, or between two successive troughs, or between two successive compressions.
