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Refraction of Light
Emmetropia and the Ametropias
Scott P. Drexler OD
University of Pittsburgh
School of Medicine
AKA: Shape is Power
Light is a Wave
The direction and quality of that wave is changed as the wave transfers from one medium to another
Interface
The boundary between two media with different indices
n n’ n n’ n n’
air water waterair glass glass
Snell’s Law
n1sinq 1 = n2sinq2
n1= index of material before refractionn2= index of material after refractionq 1= incident angle q 2= refracted angle
Snell’s Law
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.
Index of Refraction
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
Snell’s Law
Index of Refraction
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
Emmetropia
Ametropias
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.
Optical Correction
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
Concave lens- Minus power
Convex Lens-plus power
Myopia- Nearsighted
Myopia
Optical Correction
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
Hyperopia- Farsightedness
Hyperopia
Optical Correction
Convex or converging lenses
Different treatment in adults and children
Frequently treatment based on symptoms
Astigmatism
Optical Correction
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
Presbyopia- “I can’t read”
Loss of the accommodative ability of the lens that results in a difficulty focusing on near objects
Presbyopia Optical Corrections
Contact Lenses
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
Scleral Contact lenses
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
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
Lasik
Excimer Laser
Boston KPro
Boston KPro
Thank You
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