1 00:00:00,160 --> 00:00:03,600 Speaker 1: Jack Threads has quickly become the online shopping destination for guys. 2 00:00:03,680 --> 00:00:05,720 Speaker 1: Here's why everything on the side is up to eight 3 00:00:05,840 --> 00:00:08,880 Speaker 1: percent off. They serve up killer contemporary and street apparel, 4 00:00:08,920 --> 00:00:12,200 Speaker 1: accessories and gadgets from brands like Converse, Penguin, and Busted 5 00:00:12,240 --> 00:00:14,640 Speaker 1: t S. Shopping is simple and all styles are curated, 6 00:00:14,680 --> 00:00:17,119 Speaker 1: so buyer's remorse just doesn't happen. What's more, as a 7 00:00:17,120 --> 00:00:19,840 Speaker 1: listener of brain Stuff, you can skip the membership waitlist 8 00:00:19,880 --> 00:00:22,599 Speaker 1: and get instant access at sign up dot Jack Threads 9 00:00:22,640 --> 00:00:26,639 Speaker 1: dot com, slash brain Stuff. Welcome to brain Stuff from 10 00:00:26,640 --> 00:00:37,319 Speaker 1: how Stuff works dot com where smart Happens. Hi, I'm 11 00:00:37,360 --> 00:00:41,559 Speaker 1: Marshall Brain with today's question, how can my glasses change 12 00:00:41,640 --> 00:00:45,960 Speaker 1: from transparent when I'm inside too dark when I go outdoors? 13 00:00:46,840 --> 00:00:51,479 Speaker 1: Sunglasses or prescription eyeglasses that darkened when exposed to the 14 00:00:51,560 --> 00:00:54,640 Speaker 1: sun were first developed by Corning in the late nineties 15 00:00:54,800 --> 00:01:00,360 Speaker 1: sixties and popularized by Transitions in the nineteen nineties. In fact, 16 00:01:00,480 --> 00:01:04,680 Speaker 1: because of the extreme popularity of the Transitions brand, these 17 00:01:04,760 --> 00:01:09,480 Speaker 1: lenses are usually referred to as Transition lenses. The correct 18 00:01:09,560 --> 00:01:14,800 Speaker 1: term for these classes is photochromic or photochromatic, which refers 19 00:01:14,840 --> 00:01:18,640 Speaker 1: to a specific chemical reaction the lenses have to ultra 20 00:01:18,720 --> 00:01:24,959 Speaker 1: violet radiation. Photochromatic lenses have millions of molecules of substances 21 00:01:25,000 --> 00:01:29,440 Speaker 1: such as silver chloride or silver halide embedded in them. 22 00:01:29,480 --> 00:01:33,120 Speaker 1: The molecules are transparent to visible light in the absence 23 00:01:33,120 --> 00:01:36,840 Speaker 1: of UV light, which is normal for artificial lighting, but 24 00:01:36,920 --> 00:01:40,800 Speaker 1: when exposed to UV rays as in direct sunlight, the 25 00:01:40,840 --> 00:01:45,480 Speaker 1: molecules undergo a chemical process that causes them to change shape. 26 00:01:46,000 --> 00:01:49,680 Speaker 1: The new molecular structure absorbs portions of the visible light, 27 00:01:50,120 --> 00:01:53,800 Speaker 1: causing the lenses to darken. The number of molecules that 28 00:01:54,000 --> 00:01:57,279 Speaker 1: change shape varies with the intensity of the UV rays. 29 00:01:57,880 --> 00:02:00,600 Speaker 1: When you go indoors and out of the UV light, 30 00:02:00,760 --> 00:02:04,320 Speaker 1: a different chemical reaction takes place. The absence of the 31 00:02:04,400 --> 00:02:08,080 Speaker 1: UV radiation causes the molecules to snap back to their 32 00:02:08,120 --> 00:02:12,440 Speaker 1: original shape, resulting in the loss of their light absorbing properties. 33 00:02:12,800 --> 00:02:17,240 Speaker 1: In both directions. The entire process happens very rapidly. In 34 00:02:17,280 --> 00:02:21,440 Speaker 1: the original photo brown and photo gray products made by corning, 35 00:02:21,800 --> 00:02:24,560 Speaker 1: the lenses are made of glass and the molecules are 36 00:02:24,600 --> 00:02:28,639 Speaker 1: distributed evenly throughout the entire lens. The problem with this 37 00:02:28,760 --> 00:02:33,280 Speaker 1: method was apparent in prescription sunglasses, where different parts of 38 00:02:33,280 --> 00:02:37,000 Speaker 1: the lens were of varying thicknesses, the thicker parts would 39 00:02:37,000 --> 00:02:40,440 Speaker 1: appear darker than the thinner areas. But with the increasing 40 00:02:40,480 --> 00:02:44,280 Speaker 1: popularity of plastic lenses, a new method has been developed. 41 00:02:44,600 --> 00:02:48,800 Speaker 1: By immersing the lenses in a chemical bath, the photochromatic 42 00:02:48,880 --> 00:02:51,920 Speaker 1: molecules are actually absorbed to a depth of about a 43 00:02:52,000 --> 00:02:55,680 Speaker 1: hundred and fifty microns into the plastic. This is much 44 00:02:55,760 --> 00:02:58,919 Speaker 1: better than a simple coding, which would only be about 45 00:02:58,960 --> 00:03:02,200 Speaker 1: five microns thick, it would not provide enough molecules to 46 00:03:02,240 --> 00:03:07,359 Speaker 1: make the lenses sufficiently dark. This plastic lens absorption process 47 00:03:07,440 --> 00:03:13,000 Speaker 1: has been popularized by Transitions, the leading manufacturer of photochromatic lenses. 48 00:03:13,639 --> 00:03:18,160 Speaker 1: An important note about photochromic lenses. Because they react to 49 00:03:18,320 --> 00:03:22,840 Speaker 1: UV light and not divisible light, there are circumstances under 50 00:03:22,840 --> 00:03:26,440 Speaker 1: which the darkening won't occur. A perfect example of this 51 00:03:26,639 --> 00:03:30,120 Speaker 1: is in your car. Because the windshield blocks out most 52 00:03:30,240 --> 00:03:34,880 Speaker 1: UV light, photochromic lenses will not darken. For this reason, 53 00:03:35,120 --> 00:03:39,360 Speaker 1: most sunglasses with photochromic lenses also have a certain amount 54 00:03:39,360 --> 00:03:42,839 Speaker 1: of tent already applied to them. Do you have any 55 00:03:42,920 --> 00:03:46,560 Speaker 1: ideas or suggestions for this podcast? If so, please send 56 00:03:46,560 --> 00:03:49,840 Speaker 1: me an email at podcast at how stuff works dot com. 57 00:03:50,000 --> 00:03:52,240 Speaker 1: For more on this and thousands of other topics, go 58 00:03:52,360 --> 00:03:55,160 Speaker 1: to how stuff works dot com and be sure to 59 00:03:55,240 --> 00:03:57,480 Speaker 1: check out the brain stuff blog on the how stuff 60 00:03:57,480 --> 00:04:03,560 Speaker 1: works dot com home page. 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