What is the definition of amplitude in a wave?

Amplitude in the context of a wave is defined as the maximum extent of a vibration or oscillation, measured from the position of equilibrium. In simpler terms, amplitude specifically refers to the height of the wave from its midpoint (or equilibrium position) to its peak (crest) or its trough (valley).

Option B describes this concept accurately by stating that amplitude is half the distance from crest to valley. This representation captures the essence of amplitude as the maximum deviation from the equilibrium position—whether reaching up to the crest or down to the valley. The peak is the highest point of the wave, while the valley is the lowest point, and the amplitude is concerned with the maximum displacement from this central or neutral point.

To contrast with the other options, while A refers to the full distance between the crest of one wave and the crest of another, this is a measure of wavelength rather than amplitude. C speaks to frequency, which defines how many oscillations occur in a given time period but does not describe the amplitude at all. Lastly, D discusses the width of the wave instead of focusing on the height from the equilibrium position, which doesn't accurately reflect the definition of amplitude.