Lord Rayleigh found tarnished old glass let through more light than new
In 1886 Lord Rayleigh noticed something odd: slightly tarnished pieces of old glass transmitted more light than clean new ones. The dull film sat between air and glass in refractive index, splitting one strong reflection into two weaker ones. That accident is the simplest form of the anti-reflective coating now found on lenses and solar panels.
Whenever light crosses from one material into another, some bounces back; bare crown glass, the commonest optical glass, reflects about 4 percent at each surface. Anti-reflective coatings cut that loss, letting more light through and, in cameras, binoculars and telescopes, sharpening contrast by removing stray light. On eyeglasses the main gain is cosmetic, since wearers' eyes become easier to see; the claimed glare reduction is very slight. Such coatings are different from polarised sunglasses, which absorb glare bouncing off water, sand or roads.
Most coatings rely on interference. A single transparent layer works best when its refractive index equals the square root of the glass's index and its thickness is a quarter of the light's wavelength inside it, so reflections from its two faces cancel. For crown glass, index about 1.52, the ideal material would have an index near 1.23, and no solid has one that low. Magnesium fluoride, at 1.38, is the practical choice because it is cheap and durable, cutting reflection to about 1 percent. Stacks alternating silica with a higher-index material can drop it to 0.1 percent at one wavelength, and mesoporous silica films have reached an index as low as 1.12.
Nature found another route. A moth's eye is covered with hexagonally arranged bumps about 200 nanometres tall, spaced 300 nanometres apart, smaller than visible wavelengths, so light meets a smooth gradient instead of a sharp boundary. That lets the moth see at night without glinting to predators. Canon borrows the idea in a lens coating that sharply reduces flare.
Coatings also help solar cells, using materials such as silicon nitride and titanium dioxide, and chipmakers apply them in photolithography to stop reflections distorting circuit patterns.
Source: Anti-reflective coating
