Normalized frequency (fiber optics)
（重定向自V number）
In an optical fiber, the normalized frequency, V (also called the V number), is given by
where a is the core radius, λ is the wavelength in vacuum, n1 is the maximum refractive index of the core, n2 is the refractive index of the homogeneous cladding, and applying the usual definition of the numerical aperture NA.