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Homework Help: Power and focal length of human eye

  1. Sep 24, 2005 #1
    I am extremely stuck on this question and do not know how to go about it. Could somebody help me?

    1. (a) If the human eyeball is 4cm from lens to retina, what is the range of the power and focal length of the eye lens when focussing an object an infinite distance away and one at the normal near point 25cm away?

    (b) A long-sighted person(whose eyeball length is still 4cm)has a near point of 2m. What is the power of the eye lens when focussing at the near point?
    What power glasses lens would the person need to correct their near point back to 25cm?

    Any help will greatly be appreciated!
  2. jcsd
  3. Sep 24, 2005 #2


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    Homework Helper

    if the object distance is infinite, then the
    focal length *is* the image distance.
    (parallel rays converge to focus at focal length)
    Did you somehow not realize that the image
    is formed on the back of the eyeball, in focus?
    For the 25[cm] object, use the focus equation:
    ("thin-lens") 1/f = 1/d_ob + 1/d_im .

    Thin lenses held right in front of one another
    have a "total power" that's *approximately*
    the two powers multiplied together
    (ray deflection angles add).

    But another way to correct one's vision is to
    have a lens that produces a virtual image
    (of the object at 25[cm]) where the person sees
    (that is, d_im = - 200[cm] from the eye).
    This way shows that contact lenses are different
    than spectacles (with d_im = - 199[cm])
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