IEVref: 731-03-30 ID: Language: en Status: Standard Term: group index Synonym1: Synonym2: Synonym3: Symbol: N Definition: the speed of light in vacuum, divided by the magnitude of the group velocity of the mode NOTE 1 – For a plane wave of wavelength λ, it is related thus to the refractive index n: $N=n-\lambda \frac{\text{d}n}{\text{d}\lambda }$ where λ is the wavelength. NOTE 2 – Each mode has its own group index. Publication date: 1991-10 Source: Replaces: Internal notes: 2016-03-07: Editorial changes: "velocity of light in vacuum" replaced by "speed of light in vacuum". See correspondence "Questions re part 731" 2015-02 and 2015-03. JGO 2017-06-02: Cleanup - Remove Attached Image 731-03-30.gif CO remarks: TC/SC remarks: VT remarks: Domain1: Domain2: Domain3: Domain4: Domain5: