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which surface in the eye has the most power?

pre-corneal tear film (most anterior refracting surface)


how thin is the tear film?

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


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

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


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

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


what is the main optical role of the tear film?

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


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

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


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

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


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

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


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

retroillumination of pupil with slit lamp and fluorescein


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

reduction in contrast and loss of spatial resolution


how much of the eye does the cornea cover?

1/6th of the outer tunic of the eye


what is the diameter of the cornea?

11-13 mm


how much of the cornea is spherical?

central 1/3rd


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

peripheral cornea is flatter and thicker than central part


how much optical power does the cornea have?

2/3rd the optical power of the eye


what 3 things makes the corneal epithelial layer transparent?

(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


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

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


what is the corneal stroma made out of?

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


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

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


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

n fibrils = 1.55
n spacing = 1.35


why will a large corneal scar be opaque?

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


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

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


what is the total transmittance of light in the cornea?

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


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

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


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

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


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

refractive surgery = radius and thickness (d) will change
diabetes = refractive index changes with glucose levels


what is a reduced eye?

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


what are some things that pachymetry is used for?

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


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

they will have more power in their cornea


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

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