An optical device comprising a set of fine parallel slits or grooves etched onto a material in order to product interference patterns that separate components of a spectrum of incoming light. The fundamental principle is that different wavelengths diffract to different extents, so they can be separated much as a prism separates a composite polychromatic incident light beam into its constituent wavelengths of light. Human beings see this as the separation of white light into a rainbow of colored light. There are two types of diffraction gratings: