AAM refractive error & emmetropism

Question 1

refraction

Answer 1

human eye refracts at 2 ocular structures

• cornea - fixed focusing power
• lens - accommodation changes lens shape

Question 2

elements that determine eye’s ability to focus light

Answer 2

• shape of cornea
• power of lens
• length of eyeball

Answer 3

• cornea accounts for 2 thirds of total refraction of eye
• lens accounts for 1 third of refraction of eye

total power of eye = +60D
- cornea = 40D
- lens = 20D

Question 4

far point

Answer 4

the furthest distance that a person can see without the help of spectacles or contact lenses

• can be in one of three places: at infinity, at some distance in front of the eye and at some distance behind the eye

Question 5

focal point

Answer 5

the point where parallel light rays meet after passing through the lens

• can be in one of three places: macula, in front of the retina and behind the retina

Question 6

what is refractive error?

Answer 6

mismatch between the power of the eye’s optical system + the length of the eye

• results in blurred vision or eyestrain + headaches

Question 7

ametropia

Answer 7

• when light rays entering eye don’t focus on the retina
• rx not a disease but rather normal differences in visual ability`

Question 8

3 types of ametropia

Answer 8

• myopia
• hyperopia
• astigmatism

Question 8

emmetropia

Answer 8

no rx

Question 8

emmetropic eye

Answer 8

• focal point on retina
• far point at infinity (6m regarded as infinity)
• light rays from distant object are parallel
• retinal image of object on fovea

Question 9

myopic eye

Answer 9

• light rays from distant object are parallel
• focal point in front of retina
• far point in front of eye
• image formed in front of retina?

Question 10

why myopia?

Answer 10

• cornea too curved (REFRACTIVE ametropia)
• lens too powerful (RA)
• eye too long (AXIAL ametropia)
• or combination

Question 11

hyperopic eye

Answer 11

• light rays from distant object are parallel
• focal point behind retina
• far point behind eye
• image formed behind retina

Question 12

why hyperopia?

Answer 12

• cornea too flat (REFRACTIVE ametropia)
• lens too weak (RA)
• eyes too short (AXIAL ametropia)
• or combination

Question 13

astigmatism

Answer 13

• usually occurs w myopia or hyperopia
• irregular curvature of cornea/lens
• light rays focus in different locations

w the rule:
- cornea more curved in the vertical meridian

against the rule:
- cornea more curved in the horizontal meridian

Question 14

near objects

Answer 14

when looking at near objects, you must accommodate to bring object into focus

Question 15

refractive error unit of measure

Answer 15

• measured in DIOPTRES (D)
• used to describe the rx of the eye as well as the power of opthalmic lenses
• dioptre = reciprocal of distance between the lens and its focal point
• 1 / focal length (in metres)
• in eye care, we work in quarter dioptre units of power (0.25D)

Question 16

correction of rx

Answer 16

• amount of power in dioptres, needed to bring rays of light back to focus on retina
• positive, convex lenses used to converge light rays entering eye to move focal point up the plane of retina
• negative, concave lenses used to diverge the light rays to move the focal point back to plane of retina

Question 17

myopic eye

Answer 17

rays focus in front of retina

• corrected w aid of negative, concave lens to focus rays on retina

Question 18

hyperopic eye

Answer 18

rays focus behind retina

• corrected w aid of positive, convex lens to focus rays on retina

Question 19

astigmatism

Answer 19

• correct 1 meridian w sphere
• correct 2nd meridian w cylinder
  power in 1 meridian only?

Question 20

emmetropisation

Answer 20

rapid reduction in refractive error over first few years of life towards emmetropia

• co-ordination of ocular components
• human eye programmed to achieve emmetropia in childhood
• eye maintains emmetropia w advancing years

despite changes in all eye dimensions during period of growth + continuing growth of lens throughout life?

Question 21

GO THROUGH LECTURE RECORDING