Physiology 49 (The Eye: 1. Optics of Vision)
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Light rays travel slower through solids and liquids
Refractive index of transparent substance = velocity of light in air/velocity in substance (ratio)
o Velocity of light in air = 300,000 km/sec
Light rays strike interface perpendicular to beam -> don’t deviate (decreased velocity and
shorter wavelengths)
Light rays strike angulated interface -> rays bend if refractive indices are different = refraction
o Lower portion of beam enters ahead of upper edge (wave front angulated)
o The direction in which light travels is always perpendicular to the plane of the wave front
o Degree of refraction is function of 1) ratio of two refractive indices and 2) degree of
angulation
Convex lens focuses parallel light rays -> convergence
o in center, don’t refract because exactly perpendicular
o All rays bent so that they pass through single point = focal point
Concave lens diverge parallel light rays -> divergence
Convex cylindrical lens bends light rays in only 1 plane (not top or bottom) -> bent to focal line
o Concave cylindrical lens diverges light rays in only one plane
Convex spherical lens refracted at all edges (in both planes) -> rays come to focal point
2 convex cylindrical lenses at right angles -> rays come to single focal point (act like spherical
lens)
Focal length = distance beyond lens where parallel rays converge (focal point)
Diverging rays entering lens have greater focal length than parallel rays entering same lens
o Increasing curvature of lens for diverging rays can equalize focal length to that of a
parallel lens
1/f = 1/a + 1/b, where f = focal length for parallel rays, a = distance of point source of light from
lens, b = distance of focus on other side of lens
Convex lens converges point source of light on opposite side of lens directly in line with the point
source and the center of the lens
Image is upside down w.r.t. original object and lateral sides of image are reversed
Refractive power = how much a lens can bend light; measured in diopters (1m/focal length of
lens)
o Convex lens ->(+)
o Concave lens -> (-)
Concave lenses “neutralize” refractive power of convex lenses
Cylindrical lens strength must have axis stated
o Horizontal focus = 0 degrees
o Vertical focus = 90 degrees
Lens system of the eye -> refractive interfaces = 1) between air and anterior cornea, 2) between
posterior cornea and aqueous humor, 3) between aqueous humor and anterior lens, 4) between
posterior lens and vitreous humor
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Refractive indices: air (1), cornea (1.38), aqueous humor (1.33), lens (1.4), vitreous
humor (1.34)
 Total refractive power of “reduced eye” = 59 diopters (central point 17mm in
front of retina)
 Refractive power of lens = 20 diopters BUT curvature can be increased
(accommodation)
Imaged formed on retina is inverted and reversed but brain perceives it as normal
Mechanism of accommodation: suspensory ligaments attach around lens and to ciliary muscle.
Ciliary muscle has meridional fibers and circular fibers controlled by parasympathetic nerve
signals (CNIII) -> increases refractive power
o Meridional fibers extend from suspensory ligament to corneoscleral junction
(contraction pulls lens medially relaxing ligaments)
o Circular fibers arranged circularly around ligament (contraction acts like a sphincter
decreasing lens diameter relaxing ligaments)
Presbyopia = loss of accommodation by the lens with age (loss of elasticity in lens)
o Need bifocals to compensate for loss of accommodation
Major function of iris = increase/decrease amounts of light
o Amount of light entering eye through pupil is proportional to area or to square diameter
of the pupil.
Depth of focus = distance one can move away from focus on the retina and still maintain
sharpness
o Great depth of focus -> retina can be displaced considerably and image remains sharp
o Shallow depth of focus -> small retinal displacement causes extreme blurring
*Greatest depth of focus occurs when pupil is extremely small
Emmetropia = normal vision; light rays from distant objects in sharp focus when ciliary muscle is
relaxed
Hyperopia = farsightedness; eyeball too short or lens to weak so light focuses beyond retina
(ciliary muscle must contract to increase lens strength for distant object focus)
o Correct by using convex spherical lens (converge rays)
Myopia = nearsightedness; eyeball too long or strong refractive power so light focuses in front
of retina (cannot use accommodation until object comes close)
o Correct by using concave spherical lens (diverge rays)
Astigmatism = refractive error causing visual image in one plane to focus at a different distance
than that of the plane at a right angle (like an egg lying on its side)
o Light rays don’t all come to a common point (can never see clearly without glasses)
o Correct with cylindrical lens (axis and strength of lens must be determined)
 To find axis, use parallel black bars arranged circumferentially and find lens
strength that focuses 1 set of bars. Axis = parallel to out-of-focus bars
Contact lenses nullify refractive power of cornea.
o Important when cornea is odd-shaped and bulging = keratoconus
o Other advantages -> 1) lens turns with the eye (broader field of clear vision than
glasses), 2) little effect on size of object the person sees
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Cataracts = cloudy or opaque area in the lens
Retinal spot is brightest in center and about 11 micrometers diameter
o Average diameter of fovea is 1.5 micrometers so can distinguish separate points if their
centers lie up to 2 micrometers apart (must have at least 25 seconds between the
angles)
o Max visual acuity occurs in less than 2 degrees of the visual field
Expressing visual acuity = top number is distance chart is held, bottom number is clarity that
should be seen at a certain distance (ratio of an individual’s visual acuity to that of a person with
normal visual acuity)
Depth perception: 1) size of image of known objects on the retina, 2) phenomenon of moving
parallax, 3) phenomenon of stereopsis
Moving parallax -> movement of head, images of close objects move while distant objects
remain stationary (relative distance)
Stereopsis -> binocular vision allows judgment of relative distances when objects are nearby
(useless beyond 50 to 200 ft)
Ophthalmoscope = corrective lens in turret (-4 diopters for normal eyes), retina is illuminated
through pupil
Eye is filled with intraocular fluid (distends eyeball)
o Aqueous humor lines in front of lens, is free flowing fluid continually formed and
reabsorbed
o Vitreous humor is between lens and retina, gelatinous mass with proteoglycan
molecules (allows for diffusing of substances but not flow of fluid)
Aqueous humor formed at an average rate of 2-3 microliters each minute by ciliary processes
from ciliary body
o Covered by secretory epithelial cells with vascular area underneath
o Active transport of Na ions into space between epithelial cells, pulling Cl and HCO3
along causing osmosis of water
o Amino acids, ascorbic acid and glucose transported across epithelium by active
transport/facilitated diffusion
o Flows through the pupil into anterior eye chamber to anterior lens and in angle between
cornea and iris, then through trabeculae, into canal of Schlemm (empties into
extraocular aqueous veins)
 Canal of Schlemm = thin walled vein around eye (only filled with AH)
Average intraocular pressure is 15 mmHg (12-20 mmHg)
o Determined by resistance to outflow of aqueous humor into canal of Schlemm by
trabeculae
Eye pressure measured with tonometer -> cornea anesthetized and tonometer placed on it;
pushes and recorded displacement
Debris in aqueous humor (from hemorrhage or infection) may clog trabecular spaces ->
glaucoma (no reabsorption of aqueous humor)
o Phagocytic system on trabeculae cleanse them
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Glaucoma = one of most common causes of blindness; high intraocular pressure (above 25-30
mmHg can cause loss of vision when maintained); fibers and retinal artery crushed and die
o Can treat with drops that reduce secretion or increase absorption of aqueous humor