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      "utf8": "Almost 2000 years ago,"
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      "utf8": "the Roman philosopher Seneca peered \nat his book through a glass of water."
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    "segs": [ {
      "utf8": "But it wasn't until a millennium later\nthat that same principle"
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      "utf8": "Today, glasses can help millions of people\nwith poor vision"
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    "segs": [ {
      "utf8": "due to uncorrected refractive errors."
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      "utf8": "The key to understanding what that means\nlies with the term refraction,"
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    "segs": [ {
      "utf8": "the ability of a transparent medium,\nlike glass,"
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    "segs": [ {
      "utf8": "or the eye to change the direction\nof light passing through it."
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      "utf8": "The eye has two main refractive surfaces:"
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    "segs": [ {
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    "segs": [ {
      "utf8": "Ideally, these surfaces work together"
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      "utf8": "to refract light in a way that accurately\nfocuses light onto the retina,"
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      "utf8": "the layer of light-sensitive tissue\nat the back of the eye"
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      "utf8": "that works with the brain\nto give rise to vision."
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      "utf8": "But many people develop refractive errors,"
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      "utf8": "either during childhood \nas their eyes are growing,"
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      "utf8": "or in later life as their eyes age."
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      "utf8": "Imperfections in the cornea and lens"
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      "utf8": "cause refracted light to be focused\nin front of or behind the retina,"
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      "utf8": "People with refractive errors\ncan still see color,"
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  }, {
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    "segs": [ {
      "utf8": "movement,"
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  }, {
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  }, {
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    "segs": [ {
      "utf8": "but the details of what they're looking at\nare out of focus."
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      "utf8": "People experience refractive \nerrors in different ways,"
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    "segs": [ {
      "utf8": "owing to differences in their eyes."
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    "segs": [ {
      "utf8": "In some, light refracts too much,"
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  }, {
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    "segs": [ {
      "utf8": "and in others, too little."
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      "utf8": "Eyes with a focal point \nin front of the retina are called myopic,"
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      "utf8": "or short-sighted."
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      "utf8": "They can see close objects clearly,"
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      "utf8": "but those far away are out of focus."
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      "utf8": "But when the focus point \nis behind the retina,"
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    "segs": [ {
      "utf8": "people are hyperopic,\nor long-sighted."
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      "utf8": "For them, objects close up\nare unfocused,"
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      "utf8": "but distant objects are crystal clear."
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      "utf8": "Finally, some people have a cornea\nwith a non-spherical shape"
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      "utf8": "a form of out-of-focus vision\nthat makes all objects seem blurred,"
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      "utf8": "whether close or far."
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      "utf8": "As we age, our eyes face new challenges."
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      "utf8": "When we're young, the lens of the eye\nis flexible"
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    "segs": [ {
      "utf8": "Nowadays we can fix them all with glasses\nor contact lenses,"
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    "segs": [ {
      "utf8": "which work by refocusing light\nso it strikes the retina precisely."
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      "utf8": "It's even possible to correct vision\nwith surgery"
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      "utf8": "using lasers that change the shape\nof the cornea"
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      "utf8": "But glasses remain the most popular."
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    "segs": [ {
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    "segs": [ {
      "utf8": "We've come a long way \nsince Seneca's discovery"
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  }, {
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    "segs": [ {
      "utf8": "which are kept in place with arms\nwhich hook over each ear."
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      "utf8": "Today's glasses take their inspiration\nfrom that design,"
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    "segs": [ {
      "utf8": "but they're also much more precise\nand personal."
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      "utf8": "So if you're one of the 500 million people\nwith a problem with close or far vision,"
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    "segs": [ {
      "utf8": "that's hiding in plain view."
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