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Speaker 1: Welcome to Brainstuff from house stuff works dot com where

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Speaker 1: smart Happens. Hi Am Marshall Brain with today's question, how

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Speaker 1: does the anti reflective coating on a pair of eyeglasses

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Speaker 1: or sunglasses work? A common problem with prescription glasses and

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Speaker 1: sunglasses is called back glare. This is light that hits

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Speaker 1: the back of the lens and bounces into your eyes.

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Speaker 1: The purpose of an anti reflective coating is to reduce

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Speaker 1: these reflections off the lenses. In bad cases, you can

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Speaker 1: actually see the reflection of your own eyes in the lens,

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Speaker 1: similar to a scratch resistant coating. Antireflective coatings are made

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Speaker 1: of a very hard film that's very thin and is

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Speaker 1: layered on the lens. It's made of material that has

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Speaker 1: an index of refraction that's somewhere between air and glass.

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Speaker 1: This causes the intensity of the light reflected from the

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Speaker 1: inner surface and the light reflected from the outer surface

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Speaker 1: of that film to be nearly equal. When applied in

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Speaker 1: a thickness of about a quarter of a light's wavelength,

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Speaker 1: the two reflections from each side of the film basically

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Speaker 1: cancel each other out through destructive interference, minimizing the glare

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Speaker 1: that you can see off the back of the lens.

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Speaker 1: Anti Reflective coatings are also applied to the front of

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Speaker 1: prescription eyewear and some sunglasses to eliminate the hot spot

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Speaker 1: glare that reflects off the lens into other people's eyes.

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Speaker 1: While anti reflective coatings provide relief from glare off the

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Speaker 1: back of the lens, there are several ways to reduce

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Speaker 1: or eliminate glare from passing through the lenses as well.

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Speaker 1: These include, first of all, and most obviously, tinting. The

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Speaker 1: color of the tint determines the part of the light

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Speaker 1: spectrum that's absorbed by the lenses. Manufacturers use different colors

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Speaker 1: to produce specific results. Then there's polarization. Polarized filters are

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Speaker 1: most commonly made of a chemical film applied to a

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Speaker 1: transparent plastic or glass surface. The chemical compounds used will

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Speaker 1: typically be composed of molecules that naturally align in parallel

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Speaker 1: relation to each other. When applied uniformly to the lens,

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Speaker 1: the molecules create a microscopic filter that absorbs any light

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Speaker 1: matching their alignment. Then there's photochromic lenses. Sunglasses or prescription

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Speaker 1: eyeglasses that darken when exposed to the sun are called

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Speaker 1: photochromic or sometimes photochromatic, developed by Corning in the late

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Speaker 1: nineteen sixties and popularized by Transitions in the nine nineties.

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Speaker 1: Photochromic lenses rely on specifical chemical reactions to UV radiation,

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Speaker 1: and finally, there's mirroring. The lenses in these sunglasses have

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Speaker 1: a reflective coating applied in a very thin, sparse layer

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Speaker 1: on the lens. It's so thin that it's called a

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Speaker 1: half silvered surface. The name half silvered comes from the

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Speaker 1: fact that the reflective molecules coat the glass so sparsely

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Speaker 1: that only about half of the molecules needed to make

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Speaker 1: the glass and opaque mirror are applied. If you apply

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Speaker 1: all of these things together, you kind of end up

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Speaker 1: with the ultimate pair of glasses. And that's the kind

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Speaker 1: of sunglasses you see at the mall or at a

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Speaker 1: sporting goods store that cost a hundred dollars or more.

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Speaker 1: For more on this and thousands of other topics, does

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Speaker 1: that house stuff works dot com and don't forget to

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