Ch. 4 - The Ophthalmic Prescription

Front 1

What is the speed of light in air?

Back 1

186,000 mps

Front 2

Why does the speed of light travel faster through air than other substances?

Back 2

transparent substances offer more resistance

Front 3

What happens when light enters a medium at any angle other than 90 degrees?

Back 3

-the light is refracted/bent
-the amount of resistance the medium offers, along with the curvature of the surface, helps determine how much and in which direction light rays will be bent

Front 4

index of refraction

Back 4

a comparison/ration, of the speed of light in air, to the speed of light in another medium

186,000 (speed of light in air)/140,000 (speed of light in water) = 1.33 (index of refraction for water)

Front 5

index of refraction for air

Back 5

1.00

Front 6

index of refraction for water

Back 6

1.33

Front 7

index of refraction for PMMA

Back 7

1.49

Front 8

PMMA

Back 8

polymethylmethacrylate

Front 9

index of refraction for ophthalmic crown glass

Back 9

1.523

Front 10

index of refraction for flint glass

Back 10

1.616

Front 11

formula to calculate lens power in diopters

Back 11

D= 1m/focal length (m)

D= 100 cm/focal length (cm)

D= 1000 mm/focal length (mm)

Front 12

focal length

Back 12

the distance from the lens to the point where the parallel light rays come to focus

Front 13

if diopter power is known, focal length can be calculated by:

Back 13

focal length= 1m/D

focal length= 100 cm/D

focal length= 1000 mm/D

Front 14

The highter the _____ _____, the more the light rays will be bent; or the _____ _____ will be shorter.

Back 14

lens power
focal length

Front 15

What are the two basic types of ophthalmic lenses?

Back 15

plus and minus

Front 16

plus lenses

Back 16

-convex lenses
-center is thicker than the edges
-objects viewed will appear larger
-parallel light rays will be converged
-corrects hyperopia

Front 17

minus lenses

Back 17

-concave lenses
-center is thinner than the edges
-objects viewed will appear minified
-parallel light rays will be diverged
-corrects myopia
-has an imaginary focus that is found by tracing the light rays back to where they appear to be coming from in front of the lens

Front 18

Back 18

parallel light rays passing through a convex lens

Front 19

Back 19

parallel light rays passing through a concave lens

Front 20

prism

Back 20

-thought of as a triangle
-apex = point; base = bottom
-light rays passing through a prism will always be bent toward the base (does not converge or diverge, just bends the light), thus displacing, or moving, the image of objects

Front 21

prism power

Back 21

is measured by prism diopters

Front 22

the formula to calculate the power of a prism

Back 22

prism (D)=deviation of light ray (cm)/distance from prism (m)

Front 23

prism diopters

Back 23

1.00prismD will deviate a ray of light (or displace an image) 1 cm for every 1m of distance (measured on the tangent scale)

Front 24

What are the two basic lens forms used for ophthalmic prescriptions?

Back 24

spherical and cylindrical

Front 25

spherical lens

Back 25

can be thought of as a portion of a round ball; deviates rays of light in the same direction

Front 26

What are the 5 types of spherical lenses?

Back 26

-biconvex
-plano convex
-biconcave
-plano concave
-meniscus

Front 27

biconvex spherical lens

Back 27

both of the lens surfaces are plus and converge light to one focal point

Front 28

plano convex spherical lens

Back 28

has one plus surface and one plano surface and acts to converge light to a single focal point

Front 29

biconcave spherical lens

Back 29

both lens surfaces are minus and diverge light rays as if the focus was in front of the lens

Front 30

plano concave spherical lens

Back 30

has one surface that is minus and another that is plano; diverges light rays as if the focal point were in front of the lens

Front 31

meniscus spherical lens

Back 31

has one surface minus and another surface plus; it will either converge or diverge light rays, depending on which surface has more power

Front 32

cylindrical lens

Back 32

deviates light in different directions; front surface has one curved surface and one flat surface; light striking the curved surface will converge light rays, flat surface will not bring to focus (therefore has no power in one meridian and spherical plus power in the other)

Front 33

spherocylindrical lens

Back 33

has power in one major meridian and a different power in the meridian 90 degrees away

Front 34

optical cross

Back 34

-is a diagram that denotes the dioptric power in the two principal meridians of a lens
-the principal meridians are the meridians of greatest power and least power

Front 35

How far apart are the principal meridians?

Back 35

90 degrees

Front 36

total optical cross

Back 36

is a combination of the front surface power and the back surface power

Front 37

Back 37

convex/plus optical cross

Front 38

Back 38

concave/minus optical cross

Front 39

Back 39

spherocylindrical optical cross

Front 40

the horizontal meridian is located at:

Back 40

180 degrees

Front 41

the vertical meridian is located at:

Back 41

90 degrees

Front 42

Obtaining a written prescription from an optical cross:

Back 42

-spherical power=most plus, or the least minus power
-axis=meridian containing most plus or least minus power
-cylinder=subtract spherical power from the power in the meridian 90 degrees away

Front 43

transposing plus cylinder to minus cylinder

Back 43

-combine sphere + cylinder power =new sphere power
-change + symbol to - symbol
-change axis so it is 90 degrees away by either adding or subtracting 90 and making sure not to go over 180 degrees

Front 44

transpose:

+2.00 +1.00 x080

Back 44

+3.00 -1.00 x170

Front 45

sphere equivalent of an ophthalmic prescription:

Back 45

is used for a patient's prescription when the doctor desires to prescribe less than a full cylinder of power

Front 46

formula to create spherical equivalent:

Back 46

divide cylinder power in half and combine it with the sphere power

Front 47

Why is a prism normally prescribed?

Back 47

when the two eyes don't align properly

Front 48

Abbreviations for base direction of a prism:

Back 48

BO - base out
BI - base in
BU - base up
BD - base down

Front 49

lens optical center for prism

Back 49

-spot in the lens with no induced prism power
-is placed at the patient's PD

Front 50

PD

Back 50

interpupillary distance

Front 51

Prentice's rule

Back 51

-used to calculate induced prism
-rule states that induced prism is equal to the lens power multiplied by the displacement in cm of the lens optical centers from the patient's PD

Front 52

decentration

Back 52

may be utilized in order to avoid induced prism

Front 53

formula for calculating decentration

Back 53

frame PD (eye size+bridge size) - the patient's distance PD / 2
example:
64mm frame PD - 60mm patient PD = 4mm/2 = 2mm decentration, inward

Front 54

decentration is inward when:

Back 54

frame PD is LARGER than patient PD

Front 55

decentration is outward when:

Back 55

frame PD is SMALLER than patient PD

Front 56

To calculate proper multifocal power, the optometrist must:

Back 56

determine how much power the patient needs at distance and how much power the patient needs at near

Front 57

vertex distance

Back 57

the measurement from the corneal surface of the eye to the eyeglass correction

Front 58

What is the speed of light in air?

Back 58

186,000 mps

Front 59

Light waves produce vision in humans by stimulating the __________________.

Back 59

rods and cones

Front 60

The ratio of the speed of light in air to the speed of light in another medium is called the __________________.

Back 60

index of refraction

Front 61

The index of refraction 1.523 is for which of the following media?
a. ophthalmic crown glass
b. Polymethylmethacrylate
c. flint glass
d. water

Back 61

a

Front 62

What is the dioptric power of a lens whose focal length is 800mm?

Back 62

1.25D

Front 63

A _____________ lens will make objects appear larger.

Back 63

plus/convex

Front 64

A _______________ lens will make objects appear smaller.

Back 64

minus/concave

Front 65

Light rays passing through a prism will always be bent toward the _________ of the prism.

Back 65

base

Front 66

When looking through a prism, objects will appear to be displaced toward the _________ of the prism.

Back 66

apex

Front 67

When both of the lens surfaces are plus and converge light to one focal point, it is a ____________ lens.

Back 67

biconvex

Front 68

A __________________ lens has one surface that is flat, one surface that is plus, and it converges light to one single point.

Back 68

plano convex

Front 69

Most lenses used in ophthalmic prescriptions are ___________ lenses.

Back 69

meniscus

Front 70

A spherocylindrical lens has maximum power in one major meridian and a minimum power in the meridian _______ degrees away.

Back 70

90

Front 71

Ophthalmic prescriptions written by ________________ will usually be in minus sylinder form.

Back 71

optometrists

Front 72

What is the minus cylinder form of the following prescription?

OD -4.75 +5.00 x 033
OS +0.75 +0.50 x 127

Back 72

OD +0.25 -5.00 x 123
OS +1.25 -0.50 x 037

Front 73

What would be the best spherical equivalent substitute for a broken lens with a prescription of -4.25 -1.50 x 180?

Back 73

-5.00 sph

Front 74

If a patient's PD is 72 and the frame PD is 68, how much and in what direction will the laboratory need to decenter each lens in order to match the optical centers to the patient's PD?

Back 74

2mm, out

Front 75

The near correction in a multifocal lens is usually calculated for a working distance of __________ inches.

Back 75

16

Front 76

Multifocal lenses that are used to correct three working distances are called _________________.

Back 76

trifocals

Front 77

When ordering a trifocal lens, the intermediate segment will be made ______ of the near add power unless stated otherwise.

Back 77

50%