UNIT 2: Optics, Refractive Errors, Anomalies of Refraction, and Visual Acuity Flashcards

1
Q

Refractive Errors

A

An error that doesn’t permit the eye to bend light properly onto the retina.

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2
Q

Emmetropia

A

-No refractive error present
-Cornea and lens shaped correctly
-Distance between fovea and lens is correct
-Light from 20ft is focused on the retina
-The eye can accommodate for near objects
-Need no corrective lens

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3
Q

Myopia

A

-Nearsighted.
-Eyeball too long.
-Distance between lens and fovea too great.
-Light comes to focus in the vitreous humor.
-Sometimes the crystalline lens does not need to accommodate for near vision.
-A minus (diverging) lens is used to correct.

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4
Q

Hyperopia

A

-Farsighted
-Eyeball too small.
-Light from distance may or may not focus on the retina.
-Light from closer source focuses behind the retina.
-A plus, converging lens is used correct.

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5
Q

Presbyopia

A

-This is a condition, not a refractive error.
-Crystalline lens loses natural ability to focus.
-Lens loses its elasticity, ability to accommodate.
-Accommodation lessens with age.
-Bifocals, trifocals, progressive and other multifocal are used to correct.

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6
Q

Astigmatism

A

-The most common refractive error of all.
-The cornea is aspherical in the central zone.
-Light has different focal points in different meridians creating a line focus.
-Meridians are usually 90 degrees apart.
-Almost 2/3 of the population has astigmatism.
-Spherocylinder lenses are used to correct.

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7
Q

Simple Myopia

A

-Axial Myopia
-Corneal Myopia
-Lenticular Myopia

(Use minus, Spherical lenses to correct)

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8
Q

Myopia and Near Point Vision

A

-The eye has no need to accommodate and does not converge.
-The myope has a tendency to under accommodate and under converge.

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9
Q

4 ways to deal with uncorrected myopia

A

-Don’t use one eye.
-Force eyes to converge at near.
-Alternate vision.
-Eyes turn out.

(Leans toward Exophoria)

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10
Q

Simple Myopia continued..

A

-In many cases a myope has convergence without accommodation.
-This causes asthenopia and sometimes exophoria.

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11
Q

Simple Hyperopia

A

-Axial Hyperopia
-Corneal Hyperopia

(Use plus, spherical lenses to correct)

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12
Q

Hyperopia and young children

A

-Young hyperopic children can accommodate at near.
-They over-accommodate and over-converge and lean toward Esophoria.
-Great risk of becoming amblyopic if not corrected by age 6 or 7.

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13
Q

4 ways to Deal With Uncorrected Hyperopia

A

-Ignore one image, develop lazy eye.
-Diplopia
-Asthenopia
-Alternate Vision
-Eyes can become cross-eyed

(Leans toward Esophoria)

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14
Q

Simple Hyperopia

A

-Can accommodate distance vision, while trying to stop the eye from converging.
-This can lead to esophoria if not corrected, and sometimes amblyopia if left untreated.

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15
Q

Astigmia

A

-Focal line instead of focal point.
-Simple myopic astigmatism.
-Simple hyperopic astigmatism.
-Compound myopic astigmatism.
-Compound hyperopic astigmatism.
-Mixed astigmatism.

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16
Q

Gullstrand’s Model

A

-Cornea= +43.00 Diopters
-Crystalline Lens = +19.00 Diopters
-Index of refraction:
-Cornea: 1.376
-Lens: 1.416
-Aqueous/Vitreous: 1.336
-Axial Length: 24mm

17
Q

Optical Image

A

-The actual image formed by the eye’s optical system.
-It may or may not coincide with the retinal image.

18
Q

Retinal Image

A

-The image is the image actually on the retina.
-It may or may not be clear.

19
Q

Optical Image vs. Retinal Image

A

-When the image on the retina is clear, the optical image and the retinal image are the same.
-If the image on the retina is not clear (Blur Circles), then the optical image and retinal image are different.

20
Q

Blur Circles

A

The farther away the optical image is from the retina, the larger the blur circles on the retina.

21
Q

Changes that occur during accommodation

A

-The pupil constricts
-Pupillary and edge margin of iris and front surface of the lens shift forward.
-The anterior surface of the lens becomes more PLUS (there is also a slight change in the posterior surface of the lens).
-Crystalline lens sinks down slightly due to gravity.

22
Q

Donder’s table of accommodation

A

Age and Accommodation Ability:
10 - 14D
20 - 10D
30 - 7.0D
40 - 4.5D
50 - 2.5D
60 - 1.0D
70 - 0.0D

23
Q

Snellen E

A

-20/20 Line
-8.87mm Tall
-Subtend a visual angle of 1 arc minute
-Snellen/English rating system

24
Q

Other Systems for Measuring and noting Visual Acuity

A

-Decimal Rating System (1.0)
-Snellen/Metric System (6/6)

(The basis for conversion :
20/20 = 1.0 = 6/6)

25
Q

Other symbols used

A

-Big E (Snellen E)
-Konig Bars
-Landolt Ring
-Tumbling E