The distance between successive crests of a wave, which determines its color in visible light.Wavelength is the spatial period of a wave — the distance between two consecutive crests (or troughs) measured along the direction of propagation. In visible light, wavelength determines the color perceived by the human eye: red light has a longer wavelength (around 700 nanometers), while violet has a shorter one (around 400 nanometers). The concept applies to all waves, from sound and water waves to electromagnetic radiation and quantum matter waves.
The wave nature of light and the concept of wavelength were established in the 17th century by Christiaan Huygens, and later quantified by Thomas Young's double-slit experiment in 1801, which measured wavelengths of visible light.
For a wave traveling at a fixed speed, wavelength is inversely proportional to frequency: shorter wavelength means higher frequency and more oscillations per second. In electromagnetic waves, wavelength and frequency together determine the photon energy — shorter wavelength corresponds to higher energy. In quantum mechanics, the de Broglie wavelength of a particle is inversely proportional to its momentum, linking wave-like behavior to mass and velocity.
Wavelength is fundamental to spectroscopy, telecommunications (fiber optics use specific infrared wavelengths), and medical imaging (X-ray wavelengths for radiography).
The wavelength of a typical Wi-Fi signal is about 12 centimeters — roughly the width of a human hand.
| Δφ | variable | Phase shift — phase shift radian |
| λ | variable | Wavelength of light — wavelength metre |
| Δn | variable | Change in refractive index — refractive index change |
| L | variable | Path length through medium — length metre |
| d | variable | Interplanar spacing of the crystal lattice — lattice spacing metre |
| θ | variable | Glancing angle of incidence (and reflection) relative to the planes — angle radian |
| n | variable | Order of reflection (positive integer) — diffraction order |
| λ | variable | Wavelength of the incident wave — wavelength metre |
| sin | operator | Trigonometric sine of the angle — sine function |
| Δθ | variable | Rotation angle of polarization plane — polarization rotation angle radian |
| λ | variable | Wavelength of light — wavelength metre |
| RM | constant | Rotation measure, proportional to integrated electron density times magnetic field along the line of sight — rotation measure = varies (e.g., 0.812 rad m^-2 for typical radian per metre squared |
| f_λ | variable | spectral flux density per unit wavelength — spectral flux density watt per metre cubed |
| λ | variable | wavelength — wavelength metre |
| β | variable | spectral index — spectral index |
| ∝ | operator | is proportional to — proportional to |
| \Delta\lambda | variable | Line broadening (full width at half maximum) — line width metre |
| \lambda | variable | Central wavelength of the spectral line — wavelength metre |
| T | variable | Temperature of the emitting/absorbing gas — temperature kelvin |
| m | variable | Mass of the emitting/absorbing particle — mass kilogram |
| c | constant | Speed of light in vacuum — speed of light = 299792458 metre per second |
| k_B | constant | Relates thermal energy to temperature — boltzmann constant = 1.380649e-23 joule per kelvin |
| \sqrt{} | operator | Takes the square root of the argument — square root |
| E(B-V) | variable | Color excess between B and V bands — color excess |
| k(λ) | variable | Extinction curve at wavelength λ — extinction curve |
| F_λ | variable | Flux at wavelength λ — flux watt per metre squared |
| λ | variable | Wavelength — wavelength metre |
| log10 | operator | Base-10 logarithm — logarithm base 10 |
