An optical device composed of a large number of parallel slits of equal width and equal pitch is called a grating. The commonly used grating is made by engraving a large number of parallel scores on the glass sheet. The scores are opaque parts, and the smooth part between the two scores can transmit light, which is equivalent to a slit. The refined grating is engraved with thousands or even tens of thousands of scratches within a width of 1cm. This kind of grating that uses diffraction of transmitted light is called a transmission grating, and there are gratings that use diffraction of reflected light between two scores. For example, many parallel scores are carved on the surface coated with a metal layer, and the smooth metal between the two scores The surface can reflect light. This type of grating is called a reflection grating.