Wavelength is the distance from one peak of a wave to the next. In the visible spectrum it determines the hue we perceive. Shorter wavelengths appear violet or blue; longer wavelengths appear red. The human eye is sensitive to a narrow band roughly 400 to 700 nanometers.

The same physical property governs other forms of electromagnetic radiation and mechanical waves. Radio waves have wavelengths measured in meters or longer; X-rays have wavelengths far shorter than visible light. In sound, wavelength is inversely related to pitch for a given speed of propagation.

In vision research, wavelength is distinguished from the psychological experience of color. Two lights with different spectral compositions can produce identical color sensations (metamers). Conversely, the same wavelength can appear different depending on surrounding context and adaptation state.

• Distance between successive wave peaks
• Determines hue in the visible spectrum
• Inversely related to frequency
• Applies to both light and sound

Precise control of wavelength is fundamental in experiments on color vision and in technologies such as lasers and spectroscopy.