The Human Eye and Vision Optics of the eye (lenses, focusing, etc.) Physiology of the eye (detection...
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Transcript of The Human Eye and Vision Optics of the eye (lenses, focusing, etc.) Physiology of the eye (detection...
The Human Eye and Vision
Optics of the eye (lenses, focusing, etc.)
Physiology of the eye (detection of light)
Summary of Lecture
• The human eye can be modeled as a simple optical system consisting of two lenses and a projection screen for images. The cornea is the main refractive element of the eye, with the crystalline lens assisting to form an image on the retina. Rods and cones in the retina are the sensors that detect light. Common defects in the visual system are myopia, hyperopia and presbyopia. All can be corrected either with eyeglasses or, more recently for the first two, with surgical procedures.
Structure of the Eye
Diagram from National Eye Institute Website
Human Eye
Cornea – most refractionCrystalline lens – fine tuningCiliary muscles – flex the lens
Retina - extension of the brainFovea – center of retinaBlind spot - Where the optic nerve
leaves the eyeball
Human Eye - Vision Receptors
“Rods” - more sensitive to low lightlevels; fewer in vicinity of fovea.
“Cones” - bunched together at fovea;color sensitive;used for sharp images
Human Eye - Simple Model
RetinaLens
Cornea (Fine tuning)
Human Eye - Simple Model
What type of image is formed?
Real, inverted and demagnified.Inverted ???
The brain takes care of interpreting the information received at the retina.
It “compensates” for the contradictorysensory information (e.g. inversion).
Human Eye - Cornea
Most refraction occurs at the cornea(ncornea = 1.376)
Index of refraction change is greatestat that interface.
Human Eye - Goggles
Question: Why is it that you can see better underwater when wearing goggles?
Answer: Without goggles, there is very little refractive index change between the water and your cornea. You effectively become veryfar-sighted.
Human Eye - Crystalline Lens
Crystalline lens fine tunes the focus(we call this “accomodation”)
Cornea-to-retina distance is fixed
What change is needed in the lensto allow focusing of an objectas it moves closer?
The lens must take on a shorter focal length (i.e. become more converging)
“Eye Optics of Marine Mammals”
1. What first led Heather Zorn to think about marine mammal eyes?2. What is the major difference between the conditions under which
our eyes (or those of a fish) operate, and those of marine mammals?3. When you open your eyes under water without using goggles,
do you become near-sighted or far-sighted? Why is there a change at all?
4. How might the cornea play a role in marine mammal vision? The pupil? The crystalline lens? Which is most important?
Human Eye - Abnormalities
Myopia - Near-sightednessHyperopia - Far-sightednessPresbyopia - Age-related decrease in
accomodation ability
In all cases, the lens cannot compensate for all object distances.
“Normal” Eye
Retina("screen")
Object at “infinity”
Myopia: Near-sightedness
Retina("screen")
Diverging lens
Correcting Myopia
How to determine your prescription
If you are near-sighted:1. Find your “far-point” distance2. Convert this to meters3. Take the inverse of this number4. The result is your prescription, if you put a “-” in front.
Correcting Myopia
Another possibility for correcting myopia?
Radial keratotomy (RK)Photorefractive keratectomy (PRK)LASIK (Laser-assisted in situ
keratomileusis)
Hyperopia: Far-sightedness
Retina ("screen")Near point
Normal “near point” is 25cm (10")
Hyperopia: Far-sightednessNear point
Object gives “fuzzy” image because it is too close. Retina ("screen")
New image is at the near point
Correcting Hyperopia
Are there surgical options for correctinghyperopia?
Yes, LASIK can help out here as well.
Presbyopia
Age-related lack of ability to focuson nearby objects
Crystalline lens becomes less flexible
Not surgically correctable by changingthe cornea. Why?
Astigmatism
Cornea is not “spherical”Vertical and horizontal objectsfocus at different distances from the cornea.
Eyeglasses?
When do eyeglasses first appear?
Roger Bacon? (13th century)
Umberto Eco’s “The Name of the Rose”
Principles were not understood, however.
Kepler (ca. 1600) Solves the Problem
He was the first to theoretically understand how imagesare formed by lenses.Kepler still did not have an exact mathematical formulato describe the phenomenon, however.
Museum der Sternwarte Kremsmünster, Austria