The Amici prism consists of two triangular prisms in contact, with the first typically being made from a medium-dispersion crown glass, and the second a higher-dispersion flint glass. Light entering the first prism is refracted, refracted again at the interface between the two prisms, then exits the second prism at near-normal incidence. The prism angles and materials are chosen such that one wavelength (colour) of light, the centre wavelength, exits the prism parallel to (but offset from) the entrance beam. Other wavelengths are deflected at angles dependent about the dispersion of the materials. Looking at a light source through the prism thus shows the optical spectrum of the source.
Often, the prism is joined back-to-back with a reflected copy of itself. This three prism arrangement, known as a double Amici prism, increases the angular dispersion of the assembly, and has the useful property that the centre wavelength is refracted back into the direct line of the entrance beam. The prism assembly is thus a direct-vision prism, and is commonly used as such in hand-held spectroscopes.
The dispersive Amici prism should not be confused with the non-dispersive Amici roof prism.