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Speaker 1: Marshall Brain with today's question, how can my glasses change
Speaker 1: from transparent when I'm inside too dark when I go outdoors?
Speaker 1: Sunglasses or prescription eyeglasses that darkened when exposed to the
Speaker 1: sun were first developed by Corning in the late nineties
Speaker 1: sixties and popularized by Transitions in the nineteen nineties. In fact,
Speaker 1: because of the extreme popularity of the Transitions brand, these
Speaker 1: lenses are usually referred to as Transition lenses. The correct
Speaker 1: term for these classes is photochromic or photochromatic, which refers
Speaker 1: to a specific chemical reaction the lenses have to ultra
Speaker 1: violet radiation. Photochromatic lenses have millions of molecules of substances
Speaker 1: such as silver chloride or silver halide embedded in them.
Speaker 1: The molecules are transparent to visible light in the absence
Speaker 1: of UV light, which is normal for artificial lighting, but
Speaker 1: when exposed to UV rays as in direct sunlight, the
Speaker 1: molecules undergo a chemical process that causes them to change shape.
Speaker 1: The new molecular structure absorbs portions of the visible light,
Speaker 1: causing the lenses to darken. The number of molecules that
Speaker 1: change shape varies with the intensity of the UV rays.
Speaker 1: When you go indoors and out of the UV light,
Speaker 1: a different chemical reaction takes place. The absence of the
Speaker 1: UV radiation causes the molecules to snap back to their
Speaker 1: original shape, resulting in the loss of their light absorbing properties.
Speaker 1: In both directions. The entire process happens very rapidly. In
Speaker 1: the original photo brown and photo gray products made by corning,
Speaker 1: the lenses are made of glass and the molecules are
Speaker 1: distributed evenly throughout the entire lens. The problem with this
Speaker 1: method was apparent in prescription sunglasses, where different parts of
Speaker 1: the lens were of varying thicknesses, the thicker parts would
Speaker 1: appear darker than the thinner areas. But with the increasing
Speaker 1: popularity of plastic lenses, a new method has been developed.
Speaker 1: By immersing the lenses in a chemical bath, the photochromatic
Speaker 1: molecules are actually absorbed to a depth of about a
Speaker 1: hundred and fifty microns into the plastic. This is much
Speaker 1: better than a simple coding, which would only be about
Speaker 1: five microns thick, it would not provide enough molecules to
Speaker 1: make the lenses sufficiently dark. This plastic lens absorption process
Speaker 1: has been popularized by Transitions, the leading manufacturer of photochromatic lenses.
Speaker 1: An important note about photochromic lenses. Because they react to
Speaker 1: UV light and not divisible light, there are circumstances under
Speaker 1: which the darkening won't occur. A perfect example of this
Speaker 1: is in your car. Because the windshield blocks out most
Speaker 1: UV light, photochromic lenses will not darken. For this reason,
Speaker 1: most sunglasses with photochromic lenses also have a certain amount
Speaker 1: of tent already applied to them. Do you have any
Speaker 1: ideas or suggestions for this podcast? If so, please send
Speaker 1: me an email at podcast at how stuff works dot com.
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