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Emmetropia and the Ametropias
Scott P. Drexler OD
University of Pittsburgh
School of Medicine
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The direction and quality of that wave is changed as the
wave transfers from one medium to another
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The boundary between two media with different
indices
n
n’
air
water
n
air
n’
n
glass
glass
n’
water
n1sinq 1 = n2sinq2
n1= index of material before
refraction
n2= index of material after
refraction
q 1= incident angle
q 2= refracted angle
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Light travelling from a less dense to a denser material
will be refracted TOWARDS the normal.
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Light travelling from a more dense to a less dense
material will be refracted AWAY from the normal.
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In a media other than a vacuum, light waves slow down
and the wavelength also decreases v=fl
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n= Speed of light in a vacuum (C)
Speed of light in material
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Note index of refraction varies with the frequency and
wavelength of the light
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Basically- How much light is bent by a material- the
denser the material the greater the change
Since c is always the greatest(speed of light
in a vacuum), n is always greater than 1.
 It is convention to treat the nair as 1.0
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Vacuum =1
Air (nonpolluted) =1
Water= 1.33
PMMA =1.49
Crown glass =1.52
Diamond =2.417
Cornea =1.376
Zeiss hi-index =1.8
Crystalline lens= 1.42

Refractive problems, such as nearsightedness,
farsightedness, astigmatism, and presbyopia are the
result of an inability of the cornea and the lens to focus
light on the retina. Instead, light is focused either in
front of or behind the retina.

Two basic types of lenses are convex and concave. A
convex lens, also known as a plus power lens, focuses
light behind the lens; whereas, a concave lens, also
known as a minus power lens, focuses light in front of
the lens. The power of a lens is measured in Diopters
(D) and reflects the focusing distance in meters of the
lens- a + 10 D lens focuses an image at 10 cm= 1m/10D
Myopia Progression- 42% of
young adults in US are myopic
Myopia Control Treatments

Eyeglass undercorrection
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Bifocal Glasses
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Bifocal Soft Contacts
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Rigid Gas Permeable Contacts
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Orthokeratology
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Atropine
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Outdoor exposure
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Convex or converging lenses
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Different treatment in adults and children
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Frequently treatment based on symptoms
Astigmatism
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Toric lenses- may be convex in one meridian and
concave in another
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Eyeglass lens or Contact lenses

Astigmatism has both magnitude and orientation so both
glasses and contacts lenses must maintain the proper
axis
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+2.00 -1.50 X 060
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Eyeglass lens is the better optical choice

Loss of the accommodative ability of the lens that
results in a difficulty focusing on near objects

Soft Contact lenses contain the actual power of the
needed lens and drape over the cornea to correct vision

RGP contact lenses create a tear lens that functions to
correct vision so that the resulting lens power may not
be the same as the power of the eyeglass lens needed
to correct the vision
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Old technology made new again- from 1800’s
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Bypasses irregular optics of the cornea by creating a
new surface
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Also used for eye surface diseases, cosmetic problems,
and pain relief
Surgical Treatment
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Hyperopic Lasik
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Multifocal Lasik
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Intacs
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LTK
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Myopic Lasik is the most common and predictable of the
group
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Limitations of Lasik are corneal thickness and curvature
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Refractive lensectomy- cataract removal
Boston KPro
Thank You