Chromatic dispersion

So far we have treated the refractive index $n$ as a constant. In reality $n$ depends on the wavelength $\lambda$ of the incident light: this is the phenomenon of dispersion. For almost all transparent materials $n$ decreases as $\lambda$ increases, so violet is refracted more than red.

Principle — Chromatic dispersion

In a dispersive medium the refractive index depends on wavelength: $n = n(\lambda)$ For silica glass in the visible range, $n \approx 1{,}53$ for red and $n \approx 1{,}55$ for violet: a difference of about $1\%$, enough to separate the colours (Halliday, Resnick and Walker 2014).

This small dependence is the key to all chromatic phenomena of refraction: if different colours travel through glass at slightly different speeds, then each is deviated by its own angle, and white light that enters as one exits separated into a fan of colours.

Collegamenti

Argomenti: Ottica Concetti: Dispersione cromatica

Esercizi collegati: Esercizio svolto — Indice di rifrazione cromatico · Arcobaleno e dispersione · Problema — Raggi per oggetto oltre 2f (lente)