What is the difference between reflective and transmission gratings?

A grating separates polychromatic (or multiple wavelength) light into its component wavelengths by diffraction. Diffraction is a process in which light incident to a surface with dimensions similar in size to its wavelength is dispersed at certain angles. This diffraction angle is dependent upon the wavelength of light (see the grating equation above). Thus, polychromatic light will have a separate diffraction angle for each individual wavelength. This difference in diffraction angle is what separates light into its component wavelengths. In a transmission grating the diffracted light is passed through at an angle equal to the diffraction angle. For reflective gratings, the light is first diffracted by the grating and then reflected by the coating at an angle equal to the diffraction angle. Both reflective and transmission gratings follow the diffraction grating equation.