Anti-reflection coatings are a type of optical coating applied to the surface of lenses and other optical devices to reduce reflection. These coatings keep light from being lost and they prevent ghost images or flare. They are composed of transparent thin film structures, with alternating layers of contrasting refractive index. It consists of a single quarter-wave layer of transparent material whose refractive index is the square root of the substrate's refractive index. This theoretically gives zero reflectance at the center wavelength and decreased reflectance for wavelengths in a broad band around the center. These coatings are useful in situations where high transmission through a surface is unimportant or undesirable, but low reflectivity from the surface is desired.