Why do opthalmologists use plus cylinder phoropters, while optometrists use minus cylinder phoropter

Discussion in 'Optometry Archives' started by DarkProtoman, Jun 18, 2007.

  1. DarkProtoman

    DarkProtoman Guest

    Why do opthalmologists use plus cylinder phoropters, while
    optometrists use minus cylinder phoropters?

    And why are those glasses they sell in boxes at the store never minus
    power lenses; they're always plus power lenses?

    And since a minus lens has a negative diopter, and diopters are the
    reciprocal of focal length, how can a minus lens exist? That would
    mean a negative focal length, and distance can't be negative --what
    would that mean?--

    Do they make any lenses w/ a refractive index higher then 1.8? Like an
    RI of 2.00 or more?

    What would happen if a lens had an *imaginary* refractive index?

    Thank you!!!!
     
    DarkProtoman, Jun 18, 2007
    #1
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  2. DarkProtoman

    Dave Bell Guest

    Because more people need assistance in reading, thus close-up, thus plus
    diopters.
    It's a geometric thing. Instead of parallel rays being focused to a
    point a given distance past a positive lens (its focal length), the rays
    diverge, as if coming from a point some distance *beyond* the lens (its
    negative focal length.) See:
    http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/image3.html
    I don't think so, but there are people on here far more knowledgeable
    about available lens options.
    Oddly enough, all real (as in 'actual') materials *do* have an imaginary
    component to their refractive index! It's explained a little, here:

    "The refractive index of any substance is best described as a complex
    number, such as 1.34 + i 0.00067. The real part of this number is the
    "ordinary" refractive index as discussed above, whilst the imaginary
    part indicates the amount of absorption. If the imaginary part is zero,
    the substance is non-absorbing."

    http://www.philiplaven.com/p20.html
    http://en.wikipedia.org/wiki/Refractive_index

    Dave
     
    Dave Bell, Jun 18, 2007
    #2
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  3. DarkProtoman

    Churie. Guest

    Hi
    I have my own doubts as there is any difference of lens that is used
    by an Optometrist or Opthalmologist.Check on this.The optometrist
    dispenses lens for Presbyiopia and not for Myoipia or Astigmatism for
    that matter.
     
    Churie., Jun 18, 2007
    #3
  4. DarkProtoman

    p.clarkii Guest

    regardless of whether a prescription is written in plus cylinder form
    or minus cylinder form the final glasses that are dispensed are
    identical. the two forms of prescription writing (i.e. plus vs. minus
    cyl) are simply a traditional difference between optometrists and
    ophthalmologists. and yes, different phoropters are used by
    optometrists and ophthalmologists in that the JCC lens inside is
    different.

    and in the US, optometrists and ophthalmologists measure and write
    prescriptions for all types of refractive errors be it presbyopia,
    astigmatism, myopia, or whatever. and they both can treat the primary
    eye diseases as well. the difference comes in the treatment of the
    eye involving actual surgical intervention where only ophthalmologists
    are licensed to perform those therapies. this difference may not
    apply to where you live.
     
    p.clarkii, Jun 18, 2007
    #4
  5. DarkProtoman

    Salmon Egg Guest

    It seems that before being concerned about the minutia that distinguish the
    optical practices of optometrists from those of ophthalmologists, you should
    first learn some basic optics yourself.

    Bill
     
    Salmon Egg, Jun 18, 2007
    #5
  6. DarkProtoman

    DarkProtoman Guest

    OK, what if a substance had a refractive index of -1.00+0.00i? I think
    I heard about this somewhere in a science magazine. A lens made out of
    such a material would absorb no light, bend light backwards, and, if
    it was placed in a vacuum, the refractive index of the lens and the
    vacuum --which is 1.00+0.00i-- would cancel out, making the image be
    the actual object. Perfect focusing and resolution!
     
    DarkProtoman, Jun 18, 2007
    #6
  7. DarkProtoman

    p.clarkii Guest

    wow salmon egg-- i noticed your signature and i like it. its powerful
    and helps people put things into perspective doesn't it?
    so if it's about three virginia techs a month for US casualties, I
    wonder how it could be expressed for Iraqi casualties. Maybe three
    world trade centers a month.
     
    p.clarkii, Jun 19, 2007
    #7
  8. DarkProtoman

    Salmon Egg Guest

    You know, I don't care that much about Iraqi casualties. At this point I do
    not consider them my friends. There may be innocents among those casualties,
    but that is a problem the Iraqis should solve for themselves. Enough
    American blood and treasure has been unwisely spent to help what seems to be
    a helpless or antagonistic government.
     
    Salmon Egg, Jun 19, 2007
    #8
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