Theory
There are two separate causes of optical effects due to coatings, often called thick film and thin film effects. Thick film effects arise because of the difference in the index of refraction between the layers above and below the coating (or film); in the simplest case, these three layers are the air, the coating, and the glass. Thick film coatings do not depend on how thick the coating is, so long as the coating is much thicker than a wavelength of light. Thin film effects arise when the thickness of the coating is approximately the same as a quarter or a half a wavelength of light. In this case, the reflections of a steady source of light can be made to add destructively, and hence reduce reflections by a separate mechanism. In addition to depending very much on the thickness of the film, and the wavelength of light, thin film coatings depend on the angle at which the light strikes the coated surface.
Read more about this topic: Anti-reflective Coating
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