4: Optical Components of the Eye Flashcards
the cornea and tear film provide about ____ of the total refracting power of the eye
2/3
what is the tear film’s thickness and refractive index?
thickness of about 3-4 microns and refractive index of 1.337
the cornea has a thickness of about:
540 microns
what is the average radius of curvature at the vertex of the cornea for anterior surface
+7.8mm
what is the average radius of curvature at the vertex of the cornea for posterior surface
+6.3mm
what is the location and importance of the aqueous humor?
it fills the anterior chamber and posterior chamber and maintains the intraocular pressure of the eye
the anterior chamber becomes ______ on increased accommodation due to forward motion of the anterior crystalline lens surface
shallower
the iris forms the ____ _____ of the eye
aperture stop
what can the iris alter?
the retinal illuminance
the clinical “pupil” is actually the
entrance pupil
power of the lens is approximately
+19 D
the lens contains a gradient index, meaning its:
refractive index is greatest in the nucleus (about 1.43), dropping off quickly near the periphery of cortex (about 1.36)
in a relaxed lens, the thickness is
3.7 mm
the most widely accepted model for the mechanism of accommodation is attributed to
Helmholz
contraction of the annular ciliary body relaxes tension on the zonular fibers, allowing the lens to adopt a more ______ shape
curved
during positive accommodation, the equatorial diameter of the lens ______ (increases/decreases)
decreases
during positive accommodation, the lens thickness _____ (increases/decreases)
increases
during positive accommodation, the anterior chamber depth _____ (increases/decreases)
decreases
in a young eye, the power of the lens increases from about + ___ D in far vision to about +___D for an object at 10cm
from +19D to +30D
in positive accommodation, what contributes to the increased lens power?
changes in the gradient index
define presbyopia
the universal age-related loss of accommodative amplitude
define a cataract
when the transparency of the lens decreases with age and it becomes yellowed
the vitreous humor is a gel like substance, ____% water, with a refractive index of ____
98-99% water
refractive index, n=1.336
the vitreous normally contains small opacities from what
within Cloquet’s canal, which is a remnant of the former hyaloid artery
the vitreous chamber depth (VCD) averages about ____ mm in adults
16 mm
of all the ocular components, _____ is the best predictor of an individual’s refractive error
VCD
cone density is maximal at the
fovea
rod density is maximal about ___ degrees eccentricity
20 degrees eccentricity
there are no rods in the central ___ degree of the retina
1 degree
____ are important in bright (photopic) conditions
cones
____ are important in dark (scotopic) conditions
rods
the blind spot corresponds to the optic nerve head and it about ____ temporal to the fixation point in object space
15 degrees temporal
the cone receptor mosaic has what packing arrangement?
quasi-hexagonal
what defines ones upper limit on the resolution performance of the eye?
the mean inter-cone spacing
the inter-cone spacing is smallest at the
fovea (then increases peripherally)
the waveguide properties of cone and rod outer segments lead to the psychophysical phenomenon of the
Stiles-Crawford effect
define the Stiles-Crawford effect
light entering the eye from the edge of the pupil is less effectively transducer than light from the center of the pupil
what is the Stiles-Crawford effect hypothesized to reduce?
- reduces glare from intraocular light scatter
- reduce deleterious effects of ocular aberrations
Stiles-Crawford effect is hypothesized to result in slightly better ____ and ____
visual acuity and contrast sensitivity
