As humans, our usual default definition of light is the visible spectrum. However, Maxwell’s equations apply across the electromagnetic spectrum.
It is sobering the first time one realizes that germanium, a common material used in the thermal infrared spectrum, has a refractive index of approximately 4.0, resulting in a Fresnel reflection loss of 36% per surface.
In other words, without coatings, more than a third of the incoming energy can be lost at every interface.
The path of technological development is rarely a straight line, and the case of AR coatings is no different.
In the nineteenth century, both Fraunhofer and Lord Rayleigh observed that certain glass surfaces exhibited a reduced reflectance after forming a layer of tarnish with age.
In the 1890s, H. D. Taylor (perhaps best known to optical engineers as the inventor of the Cooke triplet), after noticing that tarnished lenses transmitted more light than untarnished lenses, developed a technique to artificially tarnish lenses via chemical etching.
What initially appeared to be surface degradation turned out to be a physical mechanism that could improve optical transmission.
Coatings are also used to turn what is nominally a transparent surface into a reflector (by applying a thin layer of metal) or a beamsplitter (using a coating that is designed to reflect and transmit desired percentages of the incident light).
These engineered surfaces allow optical designers to precisely control how light is distributed through an optical system.