Transcript The Human Eye and Vision

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 refraction
Crystalline lens – fine tuning
Ciliary muscles – flex the lens
Retina - extension of the brain
Fovea – center of retina
Blind spot - Where the optic nerve
leaves the eyeball
Human Eye - Vision Receptors
“Rods” - more sensitive to low light
levels; fewer in vicinity of fovea.
“Cones” - bunched together at fovea;
color sensitive;
used for sharp images
Human Eye - Simple Model
Cornea
Lens
(Fine tuning) Retina
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 contradictory
sensory information (e.g. inversion).
Human Eye - Cornea
Most refraction occurs at the cornea
(ncornea = 1.376)
Index of refraction change is greatest
at 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 very
far-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 lens
to allow focusing of an object
as 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-sightedness
Hyperopia - Far-sightedness
Presbyopia - Age-related decrease in
accomodation ability
In all cases, the lens cannot compensate
for all object distances.
“Normal” Eye
Object at “infinity”
Retina
("screen")
Myopia: Near-sightedness
Retina("screen")
Diverging lens
Correcting Myopia
How to determine your prescription
If you are near-sighted:
1. Find your “far-point” distance
2. Convert this to meters
3. Take the inverse of this number
4. 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
Near point
Retina ("screen")
Normal “near point” is 25cm (10")
Hyperopia: Far-sightedness
Near point
Object gives “fuzzy” image because it is too close.
New image is at the near point
Retina ("screen")
Correcting Hyperopia
Are there surgical options for correcting
hyperopia?
Yes, LASIK can help out here as well.
Presbyopia
Age-related lack of ability to focus
on nearby objects
Crystalline lens becomes less flexible
Not surgically correctable by changing
the cornea. Why?
Astigmatism
Cornea is not “spherical”
Vertical and horizontal objects
focus 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 images
are formed by lenses.
Kepler still did not have an exact
mathematical formula
to describe the phenomenon,
however.
Museum der Sternwarte Kremsmünster, Austria