What the wavelength symbol λ means
Wavelength is the distance between two matching points of a wave, such as one crest and the next. Physicists write it with the small Greek letter λ (lambda). It is a length, so its SI unit is the meter.
Different waves need different units. Radio waves run from millimeters to kilometers, visible light is measured in nanometers (nm), from roughly 380 nm for violet to 750 nm for red, and infrared is often given in micrometers (μm). Older books and crystallography use the angstrom, Å, equal to 0.1 nm.
λ is one of a family of wave symbols: ν (nu) or f is frequency, T is the period, k = 2π/λ is the angular wavenumber and ω = 2πf is the angular frequency. Note that ν is the Greek letter nu, not a Latin v.
The wavelength formula: λ = c/f
Wavelength, frequency and speed are tied together by λ = v/f: wavelength equals the wave's speed divided by its frequency. For light and radio waves in a vacuum, the speed is c, exactly 299,792,458 m/s or about 3.00 × 10⁸ m/s, so λ = c/f, also written c = λν.
Example: an FM radio station at 100 MHz has λ = (3.00 × 10⁸ m/s) ÷ (1.00 × 10⁸ Hz) = 3 m. A 2.4 GHz Wi-Fi signal has a wavelength of about 12.5 cm. The higher the frequency, the shorter the wavelength.
Shorter wavelengths also carry more energy per photon, as E = hc/λ shows. That is why ultraviolet light can burn skin while radio waves cannot. For particles, the de Broglie relation λ = h/p gives the wavelength of an electron or any other matter wave.
How to type λ in homework and papers
In Word and Google Docs, insert an equation and type \lambda followed by a space to get λ. Outside the equation editor in Word, type 03BB and press Alt + X. In LaTeX, write \lambda, and for subscripts such as λ₁ or λₘₐₓ use \lambda_1 or \lambda_{\max}.
In plain text, such as a chat, a slide or a spreadsheet label, copy a ready-made formula above, or build your own subscripts from the subscript symbols page. The letter itself, with its other meanings in math and coding, is covered on the lambda symbol page.