A lens with a coating that reduces reflections from its surface. When light passes between air and glass or back again, about 4–5% is reflected. By coating, or blooming, the lens with a thin film of a material such as magnesium fluoride, the reflection is reduced to 1–2%. A single coating affects light of a particular wavelength. To improve transmission across the visible spectrum, lenses are often multi-coated with several layers of different materials. The term originated from the similar appearance of a bloomed lens to the bloom on fruit such as plums.