WEBVTT - How do photochromatic lenses work?

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<v Speaker 1>how Stuff works dot com where smart Happens. Hi, I'm

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<v Speaker 1>Marshall Brain with today's question, how can my glasses change

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<v Speaker 1>from transparent when I'm inside too dark when I go outdoors?

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<v Speaker 1>Sunglasses or prescription eyeglasses that darkened when exposed to the

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<v Speaker 1>sun were first developed by Corning in the late nineties

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<v Speaker 1>sixties and popularized by Transitions in the nineteen nineties. In fact,

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<v Speaker 1>because of the extreme popularity of the Transitions brand, these

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<v Speaker 1>lenses are usually referred to as Transition lenses. The correct

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<v Speaker 1>term for these classes is photochromic or photochromatic, which refers

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<v Speaker 1>to a specific chemical reaction the lenses have to ultra

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<v Speaker 1>violet radiation. Photochromatic lenses have millions of molecules of substances

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<v Speaker 1>such as silver chloride or silver halide embedded in them.

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<v Speaker 1>The molecules are transparent to visible light in the absence

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<v Speaker 1>of UV light, which is normal for artificial lighting, but

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<v Speaker 1>when exposed to UV rays as in direct sunlight, the

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<v Speaker 1>molecules undergo a chemical process that causes them to change shape.

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<v Speaker 1>The new molecular structure absorbs portions of the visible light,

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<v Speaker 1>causing the lenses to darken. The number of molecules that

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<v Speaker 1>change shape varies with the intensity of the UV rays.

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<v Speaker 1>When you go indoors and out of the UV light,

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<v Speaker 1>a different chemical reaction takes place. The absence of the

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<v Speaker 1>UV radiation causes the molecules to snap back to their

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<v Speaker 1>original shape, resulting in the loss of their light absorbing properties.

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<v Speaker 1>In both directions. The entire process happens very rapidly. In

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<v Speaker 1>the original photo brown and photo gray products made by corning,

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<v Speaker 1>the lenses are made of glass and the molecules are

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<v Speaker 1>distributed evenly throughout the entire lens. The problem with this

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<v Speaker 1>method was apparent in prescription sunglasses, where different parts of

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<v Speaker 1>the lens were of varying thicknesses, the thicker parts would

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<v Speaker 1>appear darker than the thinner areas. But with the increasing

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<v Speaker 1>popularity of plastic lenses, a new method has been developed.

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<v Speaker 1>By immersing the lenses in a chemical bath, the photochromatic

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<v Speaker 1>molecules are actually absorbed to a depth of about a

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<v Speaker 1>hundred and fifty microns into the plastic. This is much

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<v Speaker 1>better than a simple coding, which would only be about

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<v Speaker 1>five microns thick, it would not provide enough molecules to

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<v Speaker 1>make the lenses sufficiently dark. This plastic lens absorption process

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<v Speaker 1>has been popularized by Transitions, the leading manufacturer of photochromatic lenses.

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<v Speaker 1>An important note about photochromic lenses. Because they react to

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<v Speaker 1>UV light and not divisible light, there are circumstances under

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<v Speaker 1>which the darkening won't occur. A perfect example of this

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<v Speaker 1>is in your car. Because the windshield blocks out most

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<v Speaker 1>UV light, photochromic lenses will not darken. For this reason,

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<v Speaker 1>most sunglasses with photochromic lenses also have a certain amount

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<v Speaker 1>of tent already applied to them. Do you have any

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<v Speaker 1>me an email at podcast at how stuff works dot com.

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