In an optical fiber, a dimensionless quantity, *V*, given by

where *a* is the core radius, is the wavelength in vacuum, *n*_{1} is the maximum refractive index of the core, and *n*_{2 }is the refractive index of the homogeneous cladding. *Note 1:* In multimode operation of an optical fiber having a power-law refractive index profile, the approximate number of bound modes, *i.e.,* the mode volume, is given by

where *V* is the normalized frequency greater than 5 and *g* is the profile parameter. *Note 2:* For a step index fiber, the mode volume is given by *V*^{ 2}/2. For single-mode operation, *V* < 2.405. *Synonym **V* number. 2. The ratio between an actual frequency and a reference value. **3.** The ratio between an actual frequency and its nominal value.