4: Optical Components of the Eye

Question 1

the cornea and tear film provide about ____ of the total refracting power of the eye

Answer 1

2/3

Question 2

what is the tear film’s thickness and refractive index?

Answer 2

thickness of about 3-4 microns and refractive index of 1.337

Question 3

the cornea has a thickness of about:

Answer 3

540 microns

Question 4

what is the average radius of curvature at the vertex of the cornea for anterior surface

Answer 4

+7.8mm

Question 5

what is the average radius of curvature at the vertex of the cornea for posterior surface

Answer 5

+6.3mm

Question 6

what is the location and importance of the aqueous humor?

Answer 6

it fills the anterior chamber and posterior chamber and maintains the intraocular pressure of the eye

Question 7

the anterior chamber becomes ______ on increased accommodation due to forward motion of the anterior crystalline lens surface

Answer 7

shallower

Question 8

the iris forms the ____ _____ of the eye

Answer 8

aperture stop

Question 9

what can the iris alter?

Answer 9

the retinal illuminance

Question 10

the clinical “pupil” is actually the

Answer 10

entrance pupil

Question 11

power of the lens is approximately

Answer 11

+19 D

Question 12

the lens contains a gradient index, meaning its:

Answer 12

refractive index is greatest in the nucleus (about 1.43), dropping off quickly near the periphery of cortex (about 1.36)

Question 13

in a relaxed lens, the thickness is

Answer 13

3.7 mm

Question 14

the most widely accepted model for the mechanism of accommodation is attributed to

Answer 14

Helmholz

Question 15

contraction of the annular ciliary body relaxes tension on the zonular fibers, allowing the lens to adopt a more ______ shape

Answer 15

curved

Question 16

during positive accommodation, the equatorial diameter of the lens ______ (increases/decreases)

Answer 16

decreases

Question 17

during positive accommodation, the lens thickness _____ (increases/decreases)

Answer 17

increases

Question 18

during positive accommodation, the anterior chamber depth _____ (increases/decreases)

Answer 18

decreases

Question 19

in a young eye, the power of the lens increases from about + ___ D in far vision to about +___D for an object at 10cm

Answer 19

from +19D to +30D

Question 20

in positive accommodation, what contributes to the increased lens power?

Answer 20

changes in the gradient index

Question 21

define presbyopia

Answer 21

the universal age-related loss of accommodative amplitude

Question 22

define a cataract

Answer 22

when the transparency of the lens decreases with age and it becomes yellowed

Question 23

the vitreous humor is a gel like substance, ____% water, with a refractive index of ____

Answer 23

98-99% water

refractive index, n=1.336

Question 24

the vitreous normally contains small opacities from what

Answer 24

within Cloquet’s canal, which is a remnant of the former hyaloid artery

Question 25

the vitreous chamber depth (VCD) averages about ____ mm in adults

Answer 25

16 mm

Question 26

of all the ocular components, _____ is the best predictor of an individual's refractive error

Answer 26

VCD

Question 27

cone density is maximal at the

Answer 27

fovea

Question 28

rod density is maximal about ___ degrees eccentricity

Answer 28

20 degrees eccentricity

Question 29

there are no rods in the central ___ degree of the retina

Answer 29

1 degree

Question 30

____ are important in bright (photopic) conditions

Answer 30

cones

Question 31

____ are important in dark (scotopic) conditions

Answer 31

rods

Question 32

the blind spot corresponds to the optic nerve head and it about ____ temporal to the fixation point in object space

Answer 32

15 degrees temporal

Question 33

the cone receptor mosaic has what packing arrangement?

Answer 33

quasi-hexagonal

Question 34

what defines ones upper limit on the resolution performance of the eye?

Answer 34

the mean inter-cone spacing

Question 35

the inter-cone spacing is smallest at the

Answer 35

fovea (then increases peripherally)

Question 36

the waveguide properties of cone and rod outer segments lead to the psychophysical phenomenon of the

Answer 36

Stiles-Crawford effect

Question 37

define the Stiles-Crawford effect

Answer 37

light entering the eye from the edge of the pupil is less effectively transducer than light from the center of the pupil

Question 38

what is the Stiles-Crawford effect hypothesized to reduce?

Answer 38

- reduces glare from intraocular light scatter | - reduce deleterious effects of ocular aberrations

Question 39

Stiles-Crawford effect is hypothesized to result in slightly better ____ and ____

Answer 39

visual acuity and contrast sensitivity