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