Refractive Error

Question 1

what is refractive error?

Answer 1

inability to form sharp images on retina

-Blurred image formed on the retina and glasses correct this.

Question 2

how is refractive error measured?

Answer 2

Dioptres

Question 3

what are the two types of refractive error?

Answer 3

1. spherical ametropia (sphere lens)

2. astigmatism (cylinder lens)

Question 4

what are the two types of spherical ametropia ?

Answer 4

.myopia-short sight

.hypermetropia-long sight

Question 5

what is emmetropia?

Answer 5

.sharp image formed at the retina
.power of the eye allows light to be bent and focused on the retina
- Ideal vision
-No visual defects

Question 6

what is myopia?

Answer 6

.short sight
.image is formed in front of retina
.eye is too powerful for its axial length - longer eye

Question 7

what is axial length?

Answer 7

axial length is greater than standard 22.2 mm

Question 8

what is refractive myopia?

Answer 8

power of eye is greater than standard +60D

Question 9

what is hypermetropia?

Answer 9

.long sight
.image formed behind retina
.eye is too weak for its axial length-smaller eye

Question 10

what is axial hypermetropia?

Answer 10

axial length is less than standard 22.2mm

Question 11

what is refractive hypermetropia?

Answer 11

power of eye is less than standard +60D

Question 12

how does myopic spectacle correction work?

Answer 12

.negative lens

.makes object smaller

Question 13

what is far point? Mr

Answer 13

position at which object can be seen clearly without glasses

Question 14

how do negative lens correct myopia?

Answer 14

.rays of light diverge allow image to form on right position on retina.
.creates an idea that object is formed on far point
-reduces eyes focusing power- minus lenses move the focus of light in a shortsighted eye from a point in front of the retina backward — so it falls directly onto the surface of the retina.
-uses concave lens

Question 15

how does hypermetropic spectacle correction work?

Answer 15

.positive lens

.makes object bigger

Question 16

why cant you measure Mr for hypermetropic eye?

Answer 16

far point is placed behind eye

Question 17

how do positive lens correct hypermetropia?

Answer 17

.rays of light converge
.Mr is behind eye
.lens gives the impression that the object is at that distance
-convex lens used

Question 18

what is accommodation?

Answer 18

.ability of the eye to increase its optical power with an effort to focus at near
.lens changes to increase its dioptric power
.lens thickness increases- when viewing near objects

Question 19

how does accommodation work in hypermetropic eye?

Answer 19

accommodation changes power of lens to allow image formed behind retina and place it on retina

Question 20

what is presbyopia?

Answer 20

.it is reduction in accommodation with age
.older people have no ability to accommodate
image is blurred same way as myopic person
-near vision is not good as a result

Question 21

effect of pinhole on vision?

Answer 21

.small amount of light travels through occluder
.image still forms in front of retina
.image size reduced
.less blur occurs making image clearer