Wavelength Selection : Dispersive Elements

These work by dispersing the incident radiation out spatially, creating a spectrum of wavelengths (Ingle). In a prism the diffuse radiation beam is separated because of the refractive index of the material. For example, when white light is shone onto a prism, a rainbow of colors is observed coming out the other side. This is a result of wavelength dependence on the refractive index of the prism material.

Gratings are also used to disperse incident light into component wavelengths. They work by reflecting the light off the angled grating surface, causing the wavelengths to be dispersed through constructive interference at wavelength-dependent diffraction angles (Ingle).

The condition for constructive interference (and therefore wavelength selection) on a grating surface is:

This relationship shows that the wavelength selection is not based on the grating material, but on the angle of incidence (?). The angle ? can also be used to describe the angle between normal and ?₁.