Pluck a guitar string and it moves back and forth. The distance from the string's resting position to its farthest point is the amplitude. Larger amplitude means a louder sound. That relationship holds across waves of all kinds. Amplitude measures the maximum displacement of a wave from its rest position, and it usually corresponds to the wave's energy or intensity.

In a water wave, amplitude is the height of the crest above the still water level. In a sound wave, it is the maximum pressure variation. In a light wave, it relates to brightness. A high-amplitude light wave carries more energy than a low-amplitude one. Amplitude is not the same as wavelength or frequency. Wavelength is the distance between peaks. Frequency is how often peaks pass a point. Amplitude is how tall the peaks are. Together they describe the shape and behavior of a wave.

Amplitude facts

• Definition. Maximum displacement from rest.
• Energy. Higher amplitude means more energy.
• Sound. Amplitude corresponds to loudness.
• Light. Amplitude relates to brightness.

A tsunami in deep water has small amplitude and enormous wavelength. As it approaches shore, the wavelength shrinks and the amplitude grows, which is why the wave rises so high.