(1) A set of precisely spaced, parallel lines which cause incident electromagnetic radiation to be diffracted either upon reflection or upon transmission between the lines. (2) The ruled piece of metal or other material causing diffraction. Diffraction gratings for radio waves usually consist of closely spaced wires held in a frame. Diffraction gratings for radiation at wavelengths from the infrared to ultraviolet are ruled by a graving tool on a plane or curved sheet of metal, or may be produced by photographing onto a glass plate a set of inked lines on paper. At very short wavelengths, the lattices of crystals serve as diffraction gratings.