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Today, the most commonly used laser vision correction procedure is Laser in situ Keratomileusis (LASIK) . The top layer of the cornea is surgically peeled back and the underlying tissue ablated by multiple bursts of finely controlled ultraviolet radiation produced by an excimer laser. Lasers are used because they not only produce well-focused intense light, but they also emit very pure wavelength electromagnetic radiation that can be controlled more accurately than mixed wavelength light. The 193 nm wavelength UV commonly used is extremely and strongly absorbed by corneal tissue, allowing precise evaporation of very thin layers. A computer controlled program applies more bursts, usually at a rate of 10 per second, to the areas that require deeper removal. Typically a spot less than 1 mm in diameter and about 0.3 μm in thickness is removed by each burst. Nearsightedness, farsightedness, and astigmatism can be corrected with an accuracy that produces normal distant vision in more than 90% of the patients, in many cases right away. The corneal flap is replaced; healing takes place rapidly and is nearly painless. More than 1 million Americans per year undergo LASIK (see [link] ).

The image depicts a surgeon using state-of-the-art equipment for LASIK surgery on a patient who is lying down.
Laser vision correction is being performed using the LASIK procedure. Reshaping of the cornea by laser ablation is based on a careful assessment of the patient’s vision and is computer controlled. The upper corneal layer is temporarily peeled back and minimally disturbed in LASIK, providing for more rapid and less painful healing of the less sensitive tissues below. (credit: U.S. Navy photo by Mass Communication Specialist 1st Class Brien Aho)

Test prep for ap courses

Which of the following types of light have a wavelength greater than that of visible light?

    1. gamma rays
    2. infrared
    3. radio
    4. ultraviolet
  1. I, II, and III
  2. I and IV only
  3. II and III only
  4. III only

(c)

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In LASIK surgery, a coherent UV light of 193 nm is focused on the corneal tissue.

  1. Explain the importance of using light that is all the same wavelength.
  2. Explain why UV light is more effective than infrared light at evaporating the corneal tissue.
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Section summary

  • Nearsightedness, or myopia, is the inability to see distant objects and is corrected with a diverging lens to reduce power.
  • Farsightedness, or hyperopia, is the inability to see close objects and is corrected with a converging lens to increase power.
  • In myopia and hyperopia, the corrective lenses produce images at a distance that the person can see clearly—the far point and near point, respectively.

Conceptual questions

It has become common to replace the cataract-clouded lens of the eye with an internal lens. This intraocular lens can be chosen so that the person has perfect distant vision. Will the person be able to read without glasses? If the person was nearsighted, is the power of the intraocular lens greater or less than the removed lens?

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If the cornea is to be reshaped (this can be done surgically or with contact lenses) to correct myopia, should its curvature be made greater or smaller? Explain. Also explain how hyperopia can be corrected.

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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