Transcript The Human Eye - indstate.edu

A good web site for learning much about the anatomy of the human
eye is : http://www.tedmontgomery.com/the_eye/
The Human Eye
Written for Physics 106
Friday, Nov. 7, 2008
Anatomy of the Human Eye
Leads to the
occipital
cortex at the
posterior
(back) of the
brain
Anatomy of the Eye 3D View
Refractive
index 1.38
Cataracts
occur here
Varies 2 to 7
mm
The greatest amount of refraction (~ 70%)
occurs at the air cornea boundary
Goes to the Brain
The Macula
The macula lutea is the small, yellowish
central portion of the retina. It is the area
providing the clearest, most distinct
vision. When one looks directly at something,
the light from that object forms an image on
one’s macula. A healthy macula ordinarily is
capable of achieving at least 20/201 (“normal”)
vision or visual acuity, even if this is with a
correction in glasses or contact lenses.
1 Vision sharper than this sharp may be due to there being more cones
per square millimeter of the macula than in the average eye, enabling that
eye to distinguish much greater detail than normal.
fovea centralis
The center of the macula is called the fovea
centralis, an area where all of the photoreceptors
are cones; there are no rods in the fovea. The
fovea is the point of sharpest, most acute visual
acuity. The very center of the fovea is the “foveola.”
Because the fovea has no rods, small dim objects in
the dark cannot be seen if one looks directly at
them. For instance, to detect faint stars in the sky,
one must look just to one side of them so that their
light falls on a retinal area, containing numerous
rods, outside of the macular zone. Rods detect dim
light, as well as movement.
Why do normal vision people see color?
To see any color, the retinal cone cells first must be
stimulated by light. “Red-sensitive” cones are most
stimulated by light in the red to yellow range, “greensensitive” cones are maximally stimulated by light in
the yellow to green range, and “blue-sensitive” cones
are maximally stimulated by light in the blue to violet
range. Accordingly, due to their respective
sensitivities to long (L), medium (M), and short (S)
wavelengths, they also are referred to as “L” cones,
“M” cones, and “S” cones.
Normal Vision
The process in which the lens changes its focal length to focus on objects at
different distances is called accommodation
Near point and far point
• The point nearest the eye at which an
object can be placed and still produce a
sharp image on the retina is called the
near point (~ 25 cm from the eye- 20 yr
old, 500 cm at age 60)
• The far point of the eye is the location of
the farthest object on which a fully relaxed
eye can focus. Normal vision people have
a far point of nearly infinity
myopia, hyperopia, astigmatism
If the incoming light from a far away object focuses before it
gets to the back of the eye, that eye’s refractive error is called
“myopia” (nearsightedness). If incoming light from something
far away has not focused by the time it reaches the back of the
eye, that eye’s refractive error is “hyperopia” (farsightedness).
In the case of “astigmatism,” one or more surfaces of the
cornea or lens (the eye structures which focus incoming light)
are not spherical (shaped like the side of a basketball) but,
instead, are cylindrical or toric (shaped a bit like the side of a
football). As a result, there is no distinct point of focus inside
the eye but, rather, a smeared or spread-out
focus. Astigmatism is the most common refractive error.
Nearsightedness
Eyeglasses for the nearsighted—A nearsighted person has
a far point located only 521 cm from the eye. Assume
eyeglasses ar 2 cm in front of the eye. Find the focal length
needed for the diverging lenses of the glasses needed
Farsightedness
Eyeglasses for the farsighted
presbyopia (“after 40” vision)
After age 40, and most noticeably after age 45, the human eye is
affected by presbyopia. This natural condition results in greater
difficulty maintaining a clear focus at a near distance with an eye
which sees clearly far away.
Presbyopia is caused by a lessening of flexibility of the
crystalline lens, as well as to a weakening of the ciliary muscles
which control lens focusing. Both are attributable to the aging
process.
An eye can see clearly at a far distance naturally, or it can be
made to see clearly artificially (such as with the aid of
eyeglasses or contact lenses, or else following a
photorefractive procedure such as LASIK). Nevertheless,
presbyopia eventually will affect the near focusing of every.
To find the height of the image on
the retina
di
im ageht hi
m
 
object ht ho
do
Angular Magnification
ho
 (radians) 
do
 < 9o
Magnifying Glass Physics
Remember do is related to the
thin lens equation by di and f
The astronomical telescope
The Refractor
Angular Magnification of an Astronomical
Telescope