Wavelength is the distance between two consecutive peaks (or troughs) of a wave, commonly measured in metres or nanometres. In psychology and sensation, wavelength is most often discussed in relation to light and sound.

For visible light, wavelength determines hue: short wavelengths around 400 nm appear violet, medium wavelengths appear green, and long wavelengths around 700 nm appear red. For sound, wavelength is inversely related to frequency and determines pitch: long wavelengths produce low-pitched sounds, short wavelengths produce high-pitched sounds. The amplitude of a wave, by contrast, relates to brightness for light and loudness for sound.

Key relationships

• Light wavelength → hue (colour)
• Sound wavelength → pitch
• Amplitude → brightness or loudness
• Frequency = speed of wave / wavelength