Lecture 2

Question 1

which surface in the eye has the most power?

Answer 1

pre-corneal tear film (most anterior refracting surface)

Question 2

how thin is the tear film?

Answer 2

about 10 microns (aqueous being the thickest part = 98%)

Question 3

what is the equation to find the optical power at the air-tear film interface?

Answer 3

F (power of tears) = n tears - n air/r cornea

Question 4

how big are the microvilli on the corneal epithelium? what happens when light hits them?

Answer 4

1/2 micron - same size as wavelength of light = scatters light if the refractive indices are not comparable (considered optically rough)

Question 5

what is the main optical role of the tear film?

Answer 5

to smooth the “rough surface” of the corneal epithelium and replace it with a lipid layer that is optically smooth

Question 6

why does the air-tear film interface reflect more light than any other interface in the human eye?

Answer 6

it has the worst index match = 1 vs. 1.357 (2.3% reflected)

Question 7

which interface in the eye has the best refractive index match?

Answer 7

tear film to cornea = 1.357 vs. 1.376 (0.0048% reflected)

Question 8

what is the average tear break-up time for men, women, and CL wearers?

Answer 8

men = 18 sec
women = < 18 sec
CL = > 10 sec

Question 9

which 2 techniques can give a qualitative assessment of tear film integrity?

Answer 9

retroillumination of pupil with slit lamp and fluorescein

Question 10

what 2 things can be reduced when a patient has dry eye and a physician is looking into the eye?

Answer 10

reduction in contrast and loss of spatial resolution

Question 11

how much of the eye does the cornea cover?

Answer 11

1/6th of the outer tunic of the eye

Question 12

what is the diameter of the cornea?

Answer 12

11-13 mm

Question 13

how much of the cornea is spherical?

Answer 13

central 1/3rd

Question 14

what happens in the periphery of the cornea vs. central?

Answer 14

peripheral cornea is flatter and thicker than central part

Question 15

how much optical power does the cornea have?

Answer 15

2/3rd the optical power of the eye

Question 16

what 3 things makes the corneal epithelial layer transparent?

Answer 16

(Snell’s Law) cells have homogeneous refractive index, void of opacities and pigments that absorb light, and packed tightly together with no extracellular fluid between them

Question 17

why is a edematous epithelial layer of the cornea no longer transparent?

Answer 17

the difference in refractive index between the fluid and cells redirects some of the incident light (Snell’s law)

Question 18

what is the corneal stroma made out of?

Answer 18

200 lamella (limbus to limbus - 1 to 2 microns each), each contains collagen fibrils regularly spaced

Question 19

what is the diameter and spacing of the collagen fibrils in the corneal stroma?

Answer 19

diameter = 30 nm (1/17 wavelength)
spacing = 64 nm (1/8 wavelength)

Question 20

what is the refractive index of the corneal stroma fibrils and the spacing?

Answer 20

n fibrils = 1.55

n spacing = 1.35

Question 21

why will a large corneal scar be opaque?

Answer 21

the new collagen fibrils are generally larger in diameter (5x) and arranged in a random pattern

Question 22

if the corneal stroma and sclera are both made out of collagen - why is the sclera not transparent?

Answer 22

fibrils in cornea = well defined lamella layers vs. in sclera = form irregular bundles and are thicker

Question 23

what is the total transmittance of light in the cornea?

Answer 23

91% (69% in 4 1/2 y/o and 57% in 53 y/o)

Question 24

why are there no blood vessels in the cornea, lens, or vitreous?

Answer 24

blood is a strong absorber (up to 0.6 microns) and can degrade the retinal image = blocks wavelengths 400-600nm

Question 25

what parameters of the eye influence the power of the cornea?

Answer 25

refractive index, radius of curvature, and thickness of cornea (d)

Question 26

what clinical implications do the refractive index, radius and corneal thickness have?

Answer 26

refractive surgery = radius and thickness (d) will change

diabetes = refractive index changes with glucose levels

Question 27

what is a reduced eye?

Answer 27

a type of schematic eye - only has 1 surface for cornea (no back vertex power)

Question 28

what are some things that pachymetry is used for?

Answer 28

corneal thickness, depth of anterior chamber, and location/size of pupil, FB, cataracts, etc.

Question 29

what does it mean if a certain race has a thicker cornea?

Answer 29

they will have more power in their cornea

Question 30

what are 3 ways to measure axial distances within the optics of the eye?

Answer 30

simple optical pachymeter, ultrasonic pachymetry (a-scan), and OCT

Question 31

what does the pachymeter “see” when measureing the corneal thickness?

Answer 31

an image of the posterior corneal surface

Question 32

what is the real corneal thickness compared to what is appears with a pachymeter?

Answer 32

about 1/3rd larger (image of posterior cornea is located closer to the anterior cornea and is magnified = convex mirror)

Question 33

what 2 things do you need to know to find the real corneal thickness?

Answer 33

corneal refractive index and the radius of the anterior corneal surface

Question 34

how do optometrists define the anterior chamber depth?

Answer 34

from the anterior cornea to the crystalline lens (ophthalmologists = from posterior cornea to lens - 1/2mm shallower)

Question 35

what is the perceived anterior chamber depth and what is the actual depth?

Answer 35

perceived = 3.0mm
actual = 3.5mm

Question 36

what is the perceived corneal thickness and what is the actual thickness?

Answer 36

perceived = 0.37mm
actual = 0.5mm

Question 37

how do you calculate the corneal thickness using ultrasonic pachymetry?

Answer 37

thickness = velocity x time /2 (unaffected by corneal curvature)

Question 38

what is an advantage of an ultrasonic pachymeter?

Answer 38

provides more accurate thickness measurement than optical pachymeter

Question 39

what are 2 disadvantages of an ultrasonic pachymeter?

Answer 39

separate instrument is needed and invasive (probe physically touches anterior cornea)

Question 40

what are the 4 fundamental properties of light?

Answer 40

intensity, wavelength, polarization, and coherence

Question 41

does the anterior chamber have any optical significance?

Answer 41

refractive index and depth are two key properties that affect the optical power of the eye