2015 Vision

Refraction

Critical Fact Icon The cornea and lens focus light on the retina; the cornea has greater refractive power but the focusing power of the lens can be adjusted to allow near vision (accomodation).  Refractive errors include cataracts, hyperopia, myopia, presbyopia and astigmatism.

 

  • in order to reach the photoreceptors in the retina, light passes through several transparent structures
    • the spectrum of light passed through (300-2000 nm) is greater than what can be detected by the photoreceptors (see visible light)

  • as the light passes through the cornea and the lens, the image is:
    1. inverted and reversed
    2. refracted

Eye refraction

 

Visual Pathways

Critical Fact Icon

  • objects in the LEFT VISUAL FIELD are detected by cells in the LEFT NASAL RETINA and the RIGHT TEMPORAL RETINA
  • objects in the bottom of the visual field are detected by cells in the rostral part of the retina
  • cells in the visual pathway maintain the topographic map established in the retina with ONE SIGNIFCANT MODIFICATION:
    • information from the NASAL regions of the retinas CROSS IN THE OPTIC CHIASM, ensuring that information from the left visual field is processed in the right visual cortex.
  • both the cornea and the lens have convex surfaces that bend the light and cause the image to be focused on the retina
    • the refractive power of the cornea is greater than the lens
      • the fact that the cornea is important for focusing explains why refractive surgery works
    • the power of the lens can be changed (accomodation)

  • refractive power is measured in diopters (discussed further under refractive errors)
    • the total focusing power of the eye is:
      • +44D from cornea
      • +15-+29D from lens
      • +59-+75D total

  • an eye is that is refracting normally is EMMETROPIC
Email: Dr. Janet Fitzakerley | ©2015 University of Minnesota Medical School Duluth | Last modified: 12-feb-15 6:05 AM