Welcome to Brainstuff from house stuff works dot com where smart Happens. Hi Am Marshall Brain with today's question, how does the anti reflective coating on a pair of eyeglasses or sunglasses work? A common problem with prescription glasses and sunglasses is called back glare. This is light that hits the back of the lens and bounces into your eyes. The purpose of an anti reflective coating is to reduce
these reflections off the lenses. In bad cases, you can actually see the reflection of your own eyes in the lens, similar to a scratch resistant coating. Antireflective coatings are made of a very hard film that's very thin and is layered on the lens. It's made of material that has an index of refraction that's somewhere between air and glass. This causes the intensity of the light reflected from the inner surface and the light reflected from the outer surface
of that film to be nearly equal. When applied in a thickness of about a quarter of a light's wavelength, the two reflections from each side of the film basically cancel each other out through destructive interference, minimizing the glare that you can see off the back of the lens. Anti Reflective coatings are also applied to the front of prescription eyewear and some sunglasses to eliminate the hot spot glare that reflects off the lens into other people's eyes.
While anti reflective coatings provide relief from glare off the back of the lens, there are several ways to reduce or eliminate glare from passing through the lenses as well. These include, first of all, and most obviously, tinting. The color of the tint determines the part of the light spectrum that's absorbed by the lenses. Manufacturers use different colors to produce specific results. Then there's polarization. Polarized filters are most commonly made of a chemical film applied to a
transparent plastic or glass surface. The chemical compounds used will typically be composed of molecules that naturally align in parallel relation to each other. When applied uniformly to the lens, the molecules create a microscopic filter that absorbs any light
matching their alignment. Then there's photochromic lenses. Sunglasses or prescription eyeglasses that darken when exposed to the sun are called photochromic or sometimes photochromatic, developed by Corning in the late nineteen sixties and popularized by Transitions in the nine nineties. Photochromic lenses rely on specifical chemical reactions to UV radiation, and finally, there's mirroring. The lenses in these sunglasses have a reflective coating applied in a very thin, sparse layer
on the lens. It's so thin that it's called a half silvered surface. The name half silvered comes from the fact that the reflective molecules coat the glass so sparsely that only about half of the molecules needed to make the glass and opaque mirror are applied. If you apply all of these things together, you kind of end up with the ultimate pair of glasses. And that's the kind of sunglasses you see at the mall or at a sporting goods store that cost a hundred dollars or more.
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