Refraction of Light
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Transcript Refraction of Light
Emmetropia and the Ametropias
Scott P. Drexler OD
University of Pittsburgh
School of Medicine
The direction and quality of that wave is changed as the
wave transfers from one medium to another
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
Light travelling from a less dense to a denser material
will be refracted TOWARDS the normal.
Light travelling from a more dense to a less dense
material will be refracted AWAY from the normal.
In a media other than a vacuum, light waves slow down
and the wavelength also decreases v=fl
n= Speed of light in a vacuum (C)
Speed of light in material
Note index of refraction varies with the frequency and
wavelength of the light
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
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
Bifocal Glasses
Bifocal Soft Contacts
Rigid Gas Permeable Contacts
Orthokeratology
Atropine
Outdoor exposure
Convex or converging lenses
Different treatment in adults and children
Frequently treatment based on symptoms
Astigmatism
Toric lenses- may be convex in one meridian and
concave in another
Eyeglass lens or Contact lenses
Astigmatism has both magnitude and orientation so both
glasses and contacts lenses must maintain the proper
axis
+2.00 -1.50 X 060
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
Old technology made new again- from 1800’s
Bypasses irregular optics of the cornea by creating a
new surface
Also used for eye surface diseases, cosmetic problems,
and pain relief
Surgical Treatment
Hyperopic Lasik
Multifocal Lasik
Intacs
LTK
Myopic Lasik is the most common and predictable of the
group
Limitations of Lasik are corneal thickness and curvature
Refractive lensectomy- cataract removal
Boston KPro
Thank You