Optics and the Contact Lens

Question 1

Describe the basic movement of light

Answer 1

Light travels outward from a source in concentric rings. As the rings get further away from a source, they become flatter. When the rings reach 20 feet away from the source, they appear flat which means they appear to originate at infinity

Question 2

What is Vergence?

Answer 2

Vergence refers to the amount of curve that the ring of light has. It can be convergent or divergent depending on the power design of the lens

Question 3

Describe convergent lenses

Answer 3

Plus power, two prisms base to base so they are thicker in the middle, magnifies images so it shortens the focal length enabling the image to land on the retina

Question 4

Describe Divergent lenses

Answer 4

Minus power, two prisms stacked apex to apex so it is thinner in the middle, minifies images so it lengthens the focal length enabling the image to land on the retina

Question 5

Describe Effective Power

Answer 5

Effective power is described as a ray of light traveling from a focal point having variances of power along the way at any given point.

Question 6

What is the index of refraction

Answer 6

A ratio of a ray of light changes speed within a chosen medium or material. The greater the change in speed, the greater the index of refraction becomes. It is a measure of how the speed of light in air is compared to the speed of light in a chosen material

Question 7

What is the formula for calculating index of refraction?

Answer 7

n=speed of light in air/speed of light in selected material

Question 8

What is the speed of light in air?

Answer 8

186,000 miles per second

Question 9

What is the Index of Refraction of Air?

Answer 9

1.00

Question 10

What is the Index of Refraction of the Cornea?

Answer 10

1.37

Question 11

What is the Index of Refraction of the Crystalline Lens?

Answer 11

1.42

Question 12

What is the index of refraction of the aqueous and vitreous?

Answer 12

1.34

Question 13

What is the radius of curvature of the anterior cornea?

Answer 13

7.7

Question 14

What is the radius of curvature of the posterior cornea?

Answer 14

6.8

Question 15

What is the radius of curvature of the anterior lens cortex?

Answer 15

10.0

Question 16

What is the radius of curvature of the posterior lens cortex?

Answer 16

-6.0

Question 17

What is the radius of curvature Anterior lens nucleus?

Answer 17

7.91

Question 18

What is the radius of curvature of the posterior lens nucleus?

Answer 18

-5.76

Question 19

What is the corneal thickness of the eye?

Answer 19

0.5mm

Question 20

What is the axial length of the posterior cornea to the anterior crystalline lens?

Answer 20

3.1mm

Question 21

What is the thickness of the crystalline lens?

Answer 21

3.6mm

Question 22

What is the axial length from the posterior crystalline lens to the retina?

Answer 22

17.2 mm

Question 23

What is the total Axial length of the eye?

Answer 23

24.4mm

Question 24

Define Emmetropia

Answer 24

Refers to an eye free of refractive errors. This means the light entering the eye is focused directly on the retina

Question 25

Define Myopia

Answer 25

Myopia occurs if the axial length is longer than normal or the curve of the cornea is too steep, causing light rays to focus in front of the retina.

Question 26

Define Hyperopia

Answer 26

Hyperopia occurs if the eye is too short or the curve of the cornea is too flat. Some hyperopes still achieve clear vision by using accommodation to compensate.

Question 27

Define Astigmatism

Answer 27

Astigmatism occurs when the cornea has an oblong shape in one or more directions. These directions are axes, or principle meridians and are typically 90 degrees apart. The axes cause the light entering the eye to split, allowing light to focus on more than one point.

Question 28

Define Simple Myopic Astigmatism

Answer 28

One point comes into focus in front of the retina, and one point comes into focus directly on the retina.

Question 29

Define the type of Astigmatism in this RX: Plano -1.00 x 180 transposed to -1.00 +1.00 x 090

Answer 29

Simple myopic astigmatism, if one of the transposed RX's are plano, it is a simple astigmatism.

Question 30

Define Simple Hyperopic Astigmatism

Answer 30

One point comes into focus directly on the retina, and one point comes into focus behind the retina

Question 31

Define the type of astigmatism in this RX: +4.25 -4.25 x 090 transposed to Plano +4.25 x 180

Answer 31

Simple Hyperopic Astigmatism, If one of the transposed RX's are plano, it is a simple astigmatism

Question 32

Define Compound Myopic Astigmatism

Answer 32

Both points of light come into focus directly on the retina.

Question 33

Define the type of astigmatism in this RX: -3.00 -2.00 x 090 transposed to -5.00 +2.00 x 180

Answer 33

Compound myopic astigmatism, since there is a minus sphere in both forms of the prescription

Question 34

Define Compound Hyperopic Astigmatism

Answer 34

Both points of light come into focus behind the retina

Question 35

Define the type of astigmatism in this RX: +3.00 -2.00 x 090 transposed to +1.00 +2.00 x 180

Answer 35

Compound hyperopic astigmatism since there is a plus sphere in both forms of the prescription

Question 36

Define Mixed Astigmatism

Answer 36

One point of light comes into focus in front of the retina, and one point comes into focus behind the retina

Question 37

Define the type of astigmatism in this RX: -0.75 +1.25 to 090 transposed to +0.50 -1.25 x 180

Answer 37

Mixed astigmatism since there is a plus and a minus sphere in both forms of the prescription

Question 38

Define Coma

Answer 38

An lens abberation that gets its name from the flare shape, usually eliminated by the pupil

Question 39

Define Marginal or Oblique Astigmatism

Answer 39

The optical aberration astigmatism is when two images refracted by the lens break into two seperate planes. Images can be skewed depending on the vertex distance of the object being viewed.

Question 40

Define Spherical Aberrations

Answer 40

Occurs because the rays of light are bent more at the periphery than they are closer to the optical center. The result is a blurry image at the edge of the lens because the rays do not form a single point.

Question 41

Define Curvature of Field

Answer 41

Also called power error, curvature of field is caused because light does not come into focus on a flat plane. It must be curved like the retina to accommodate the way the focal plane falls.

Question 42

Using the magnification formula, why is d equal to d plus 3 mm?

Answer 42

3mm is the distance from the edge of the backside of the cornea to the pupil

Question 43

What is another name for the power of a contact lens?

Answer 43

Back-vertex power

Question 44

How is the back vertex power of a contact lens determined?

Answer 44

by the radius of curvature of the front and back surfaces of the lens, the index of refraction of the material and the thickness of the material

Question 45

What are constants to remember when figuring the power of the lacrimal lens?

Answer 45

-the index of refraction of tears is1.3375 -center thickness is so thin it is considered irrelevant - The front surface of the lacrimal lens is the same curvature as the back side -If the contact lens curvature and the cornea curvature are the same, the power of the lacrimal lens is plano

Question 46

When calculating the power of the lacrimal lens, what are things to keep in mind while looking at "K" readings?

Answer 46

1. The lacrimal lens will always have the same cylinder power and axis as the anterior cornea 2. When the curvature of the contact lens is equal to the flattest "K", the lacrimal lens sphere power is plano 3. If the contact lens curvature is steeper than the flattest "K", the lacrimal lens sphere power is plus 4. If the contact lens curvature is flatter than the flattest "K", the lacrimal lens sphere power is minus

Question 47

How do you choose whether to add SAM or FAP when finding the power of the lacrimal lens?

Answer 47

If the base curve of the lens is steeper than the flattest "K", then the minus should be added to the sphere power (steeper add minus). If the base curve of the lens is flatter than the flattest "K", then the plus should be added to the sphere power (flatter add plus) This adjusts for the plus or minus power of the lacrimal lens

Question 48

How do you determine the amount of residual astigmatism when fitting soft contact lenses?

Answer 48

A contact lens fitter will be able to determine if there is lenticular astigmatism if there is a difference between the amount of astigmatism indicated by "K" readings and the refraction

Question 49

How do you determine the amount of cylindrical power incorporated into a toric soft contact lens?

Answer 49

The amount of cylinder should be the same as the residual error and will not correlate to the spectacle cylinder power. This would be incorporated into the front of the lens, creating a front surface toric lens. The back surface is the fitting side to the eye and should not be changed.

Question 50

What happens if the amount of power required for fitting is the same as needed for correction?

Answer 50

The contact lens fitter would select a spherical front surface with a cylindrical back surface lens design.

Question 51

What happens if the amount of power required for fitting is different than the amount of power needed for correction?

Answer 51

The contact lens fitter would select a lens with cylinder power on both surfaces of the lens (called a bitoric lens)

Question 52

During CL fitting, why will the powers created by the corneal meridians not be equal to the power created by the curves ground into the lens?

Answer 52

Because of the differences in the index of refraction of the eye and the cornea.

Question 53

What is the most common index of refraction for material used in contact lenses today?

Answer 53

1.47

Question 54

The difference between the corneal cylinder and the contact lens cylinder can be seen by multiplying the corneal cylinder by what?

Answer 54

1.39

Question 55

What are the steps for determining power?

Answer 55

1. Converting rx into minus cylinder if necessary 2. Compensate for vertex distance for any power 4.00D or greater 3. If the base curve is steepened or flattened in equal measure on both meridians, adjust the sphere power using SAM or FAP. 4. If one curve is flattened or steepened more than the other, the lacrimal lens power changes the cylinder power of the contact

Question 56

What is the difference between front toric soft lenses and back toric soft lenses?

Answer 56

Front toric lenses have little corneal cylinder or residual astigmatism, and back toric lenses have high corneal cylinder.

Question 57

What is the contact lens over refraction for distance process?

Answer 57

1. Select a lens as close to the desired power as possible 2. Ensure that the eye is in a relaxed state without any accommodation 3. Using a diagnostic lens more plus than needed, no accommodation will occur 4. Using a diagnostic lens more minus than needed, accommodation is stimulated 5. If accommodation is stimulated, fog the patient's vision using a couple diopters of extra power and after removing it immediately test vision.

Question 58

Why is convergence a lot easier for contact lens wearers that are hyperopes?

Answer 58

Because the base out prism created by their convergence in spectacles is missing. This is much harder for myopes, they may need additional power in their contact lenses to assist with accommodation.

Question 59

What does LARS stand for and what is the purpose?

Answer 59

LARS stands for Left Add, Right Subtract. It is used to ensure that the lens remains on axis even during rotation. If the lens is seen to rotate to the left, add that amount of rotation to the spectacle axis; if it rotates to the right you would subtract it.