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dot com, slash brain Stuff. Welcome to brain Stuff from how Stuff works dot com where smart Happens. Hi, I'm Marshall Brain with today's question, how can my glasses change from transparent when I'm inside too dark when I go outdoors? Sunglasses or prescription eyeglasses that darkened when exposed to the sun were first developed by Corning in the late nineties
sixties and popularized by Transitions in the nineteen nineties. In fact, because of the extreme popularity of the Transitions brand, these lenses are usually referred to as Transition lenses. The correct term for these classes is photochromic or photochromatic, which refers to a specific chemical reaction the lenses have to ultra violet radiation. Photochromatic lenses have millions of molecules of substances such as silver chloride or silver halide embedded in them.
The molecules are transparent to visible light in the absence of UV light, which is normal for artificial lighting, but when exposed to UV rays as in direct sunlight, the molecules undergo a chemical process that causes them to change shape. The new molecular structure absorbs portions of the visible light, causing the lenses to darken. The number of molecules that change shape varies with the intensity of the UV rays. When you go indoors and out of the UV light,
a different chemical reaction takes place. The absence of the UV radiation causes the molecules to snap back to their original shape, resulting in the loss of their light absorbing properties. In both directions. The entire process happens very rapidly. In the original photo brown and photo gray products made by corning, the lenses are made of glass and the molecules are
distributed evenly throughout the entire lens. The problem with this method was apparent in prescription sunglasses, where different parts of the lens were of varying thicknesses, the thicker parts would appear darker than the thinner areas. But with the increasing popularity of plastic lenses, a new method has been developed. By immersing the lenses in a chemical bath, the photochromatic molecules are actually absorbed to a depth of about a
hundred and fifty microns into the plastic. This is much better than a simple coding, which would only be about five microns thick, it would not provide enough molecules to make the lenses sufficiently dark. This plastic lens absorption process has been popularized by Transitions, the leading manufacturer of photochromatic lenses. An important note about photochromic lenses. Because they react to UV light and not divisible light, there are circumstances under
which the darkening won't occur. A perfect example of this is in your car. Because the windshield blocks out most UV light, photochromic lenses will not darken. For this reason, most sunglasses with photochromic lenses also have a certain amount of tent already applied to them. Do you have any ideas or suggestions for this podcast? If so, please send me an email at podcast at how stuff works dot com.
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