Anti-reflective coatings are applied to lenses and other devices to reduce reflection from optical surfaces. This often improves the efficiency of the system; this is especially important if the light wasted by reflections is difficult to obtain, i.e. from a large telescope, an obstacle on a dark roadway, or an intricate optics experiment. They also have qualitative benefits like making the eyes of a glasses-wearer more visible and reducing the glint from a sniper's scope.

Often, they are composed of transparent thin film structures, with alternating layers of contrasting refractive index. Layer thicknesses are chosen to produce destructive interference in the beams reflected from the many interfaces, and constructive interference in the corresponding transmitted beams. This makes the structure's performance change with wavelength and incident angle (as in diffraction), so that color effects often appear at oblique angles. A wavelength range must be specified when designing or ordering such coatings, but good performance can often be achieved for a relatively wide range of frequencies: usually a choice of IR, visible, or UV is offered.