2ndSem.-Physics – LO Lab-06
The Amazing Human Eye
LO - Lab #06 - The Amazing Human Eye
In this lab you will examine and model one of the most amazing optical systems you
will ever encounter: the human eye. You might find it helpful to review the anatomy and
function of the human eye before you begin.
Equipment:
Your eyes, Eye charts, Human eye model, Light box, Lens box containing: a 13mm aperture, spherical lenses with the powers +2.0, +7.0. +20.0 and -1.75 (all
measured in diopters), and cylindrical lenses with the powers of +1.75 and -5.50.
Activity #1: What Can (and Can't) You See?
Complete the following exploration activities with your own eyes. Everyone should do
each activity and record your results in your logbook. You can do the activities in any order,
however, be sure to clearly state what you did in your descriptions in your logbook.
(a) How Do Your Eyes Accommodate?
Look up from this paper at an object somewhere across the room and then look back at
the paper. Were you able to clearly see both the other side of the room and the paper?
This activity illustrates the process of accommodation, or focusing, for your eyes.
Accommodation is automatically accomplished in your eye by a set of muscles that changes the
curvature of the crystalline lens. (See figure on page 3 for details.) That is, the eye actually
changes the shape of the lens.
1.
Think of the glass lenses you used last week in lab. Were they able to produce clear
images for any and all object distances or were they constrained by certain parameters?
What does the ability of your eye to accommodate tell you about the focal length of your
eyes' lenses?
(b) What is Your Visual Acuity?
You can measure your own visual acuity. Stand at the taped marker in the hallway and
look at the eye chart hanging on the wall. Only one group should do this activity at a time. If a
group is already working with this equipment, just move on to the following explorations and
come back to this later.
A person whose vision is rated 20/20 is able to see details at a distance of 20 feet as
clearly as a "normal" individual would. A rating of 20/15 is better than average for at 20 feet
the person can see details that would be clear for "normal" vision at 15 feet. When visual acuity
falls below 20/200, the individual is considered to be legally blind.
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The Amazing Human Eye
2.
Which lines can you read clearly? What is your visual acuity at twenty feet? Describe in
your own words what this rating means.
3.
Do you think this test is as good as an eye test at the doctor's office? If not, what things do
you think would have to be improved to make it a better test?
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The Amazing Human Eye
(c) Where is Your Near Point?
Next, measure your near point. That is, measure how close you can hold this page to
your eyes and still see it clearly. The typical near point for people is 25 cm (though it can be
much closer when you are young). The nearpoint is the point that can be seen clearly with
maximum accomodation of your eyes. Record the distance of your nearpoint.
4.
(a)
Recall that the ciliary muscles determine the curvature of the eye's crystalline lens.
How are the muscles changing your lens to allow you to see things this close? That
is, do you need the lens to have more or less curvature to see near objects?
(b)
Why do you think your nearpoint changes with age? Discuss your ideas.
(d) Your Blind Spot
The optic disc is the region of the back of your eye where the optic nerve originates.
There are no light detectors on this disc. Because light striking this area goes unnoticed, it is
commonly called the "blind spot." You do not usually notice a blank spot in your visual field
because involuntary eye movements keep the visual image moving and allow the brain to “fill
in” in the missing information.
Locating your blind spot:
5.

Close your left eye and stare at the cross on Eye Chart #2 with your right eye, keeping it
in the center of your field of vision. Begin with the page a few inches away and
gradually increase the distance as you keep staring at the cross. Note how far the paper
needs to be away from your eyes to have the dot "disappear." Watch what happens if
you continue to move it further away.

Repeat this activity by closing your right eye and staring at the dot with your left eye.
How far away does the paper have to be before the dot disappears and then reappears for
each of your eyes?
(e) Astigmatism
Astigmatism is usually caused when the cornea or lens is out-of-round. This common
defect causes point-like objects to focus as lines and therefore blurs the image.
Test your own eyes for astigmatism using the figure in Eye Chart #2. Look with one eye
at the center of the pattern. Sharply focused lines appear dark and those that are not in focus
appear dimmer or gray.
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The Amazing Human Eye
Record your observations for your own eyes.
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The Amazing Human Eye
Activity #2: Preparing The Eye Model
Now that you have explored some of the remarkable properties of your own eyes, you
will model the physical properties of the human eye with the provided human eye model.
Here is a brief description of the parts of the human eye along with their counterparts in
the eye model.
Image of a Human Eye
Part of a
Human Eye
Image of the eye model you will use in lab
General Description
Cornea
The first and most powerful lens
of the eye’s optical system
Iris
Controls the amount of light
intensity that enters the eye’s
optical system
Pupil
The variable opening in the iris
Crystalline Lens
Second lens of the eye’s optical
system
Ciliary Muscle
Muscles controlling the curvature
of the crystalline lens
Vitreous Humor
Clear colorless jelly that fills the
eyeball
Retina
Light sensitive membrane
distributed over the back of the
eyeball
Fovea
The most sensitive region of the
Part of the
Eye Model
Meniscus lens C
(Fixed in the eye model)
Aperture insert
(Placed at position G1)
Aperture insert
(Placed at position G1)
Lens insert
(Placed at position L)
The eye model is filled
with water.
Curved screen
(Placed at position R)
Dashed markings on
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Optic Nerve
The Amazing Human Eye
retina
the curved screen.
Conducts visual stimuli to the
brain
Shown as the spot on
the curved screen.
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The Amazing Human Eye
The power of a lens is often measured in the unit of diopters (for instance, eyeglass
prescriptions are given in units of diopters). The power of a lens is computed by taking the
reciprocal of its focal length when the focal length is measured in meters.
Lens power (diopters ) =
6.
1
f (m)
Compute the power of the two converging lens(es) that you used in the last lab. If a lens
has a higher power, then does it have a longer or shorter focal length?
Remove any lenses that may have been left in the model from the last class (positions L ,
G and S). Verify that the curved screen, which simulates the eye’s retina, is placed in the
“normal” position (R). That is, place the screen in the middle of the three possible positions.
Take your eye model into the hallway and carefully fill it with water at the sink before
doing any of the following activities. Fill it so the model’s cornea is completely covered, but
don’t fill it so full that water spills over the top.
7.

Please be careful not to spill water in the hallway.

If any water is spilled, please notify your lab instructor right away so it can be
cleaned up before anyone slips.
Why do you think you use water in your eye model? How does this water relate to the
human eye? What physical properties might it simulate?

Turn off the overhead lights before you continue so you can best judge when
images are in focus.
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The Amazing Human Eye
Activity #3: Modeling Accommodation with the Eye Model
Looking at far away objects…
•
Set up the model so that it is "looking" toward an unshaded window or other bright object
4 or 5 meters away. Use an object with features that you can recognize in the image (like
your lab partner standing in front of the window). Don't use a bare light bulb, which only
looks like a bright spot and does not have any distinguishing features.
•
Describe the object and describe precisely how the image looks on the retina. Comment on
important features like the size of the image, if it is right side up or upside down, etc.
•
Find a spherical lens to insert into the groove L that gives a clear, sharp image of the far
away object on the retina.

Record both the power of this lens in diopters and its focal length in meters. Describe how
the new image looks on the retina. Note the characteristics of the image including: whether
it is erect or inverted, the image size compared to object size, ...
Looking at near objects…
•

8.
Without changing anything in the eye model, turn the model so it is looking at a near object
(namely, the light box). Position the light box with the radially-slotted pattern 35 cm in
front of the model’s cornea.
Sketch the image of the light box on the retina and describe how it looks.
How does the quality of the image compare to the image that was formed when the model
was looking far away?
Accommodating for near objects…
•
•
Replace the crystalline lens with one that makes the image of this near object clear.
Record your observations and lens choice. Carefully describe this image along with any
notable characteristics.
9.
How does the crystalline lens needed for the model to have clear far vision compare to the
lens needed to clearly view near objects? Is this in agreement with your answer to
question #4a?
10.
How does the image that is formed on your retina differ from what your brain tells you
that you are seeing? Explain.
11.
How does the process of accommodation for your real eyes compare to and differ from the
process of accommodation of the eye model?
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The Amazing Human Eye

During the remainder of this lab, you will be modeling the eye's function when
it is looking at near objects. Therefore, you must leave the crystalline lens for
near vision in place (position L) for the remainder of the lab.

Feel free to verify that you have the correct lens by comparing with another
group or asking your lab instructor.
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The Amazing Human Eye
Activity #4: Farsighted and Nearsighted Vision
Two of the most-common defects that occur with human vision are farsightedness and
nearsightedness. These two conditions are briefly defined here.
Farsightedness
Someone with farsighted vision is only
able to clearly see objects far away.
Farsightedness (hypermetropia) occurs if a
person’s eyeball is "short." This results in
parallel light being focused behind the retina.
Nearsightedness
Someone with nearsighted vision is
only able to clearly see near objects.
Nearsightedness (myopia) occurs if a person’s
eyeball is "long." This results in parallel light
being focused in front of the retina.
Becoming an optometrist for an eye model…





12.
Set aside your model of a "normal" eye from Activity #3.
Your lab instructor has two patients, Martha and George, who are in need of eyeglasses.
Request a patient from the instructor so you can complete the following activity.
If you find that a patient is already busy with another doctor, then you can continue on to
Activity #5 until the patient is available.
Be sure to return the patient to your instructor once you have finished.
Complete a study of your patient as he/she is looking at the light box from a distance of
35 cm. As part of this study, be sure to answer all of the following questions. Explain
the evidence that led you to your conclusions.






Record your patient's name.
Is this patient nearsighted or farsighted?
What impact does their visual defect have on their ability to form a clear image?
Give a careful description and/or sketch.
What shape of lens is needed to correct this defect?
Using the lenses provided in your box, find an appropriate lens to correct this
patient's vision. Keep in mind that you are only licensed to determine a prescription
for this patient (position S1), you are not licensed to do surgery! (That is, do not
remove the lens L!)
Make a note of your prescription and the resulting image formation.
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13.
The Amazing Human Eye
Repeat this activity for the second patient, answering the same questions listed in #12.
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The Amazing Human Eye
Activity #5: Modeling Astigmatism with the Eye Model
In the human eye, astigmatism is generally caused by a slight cylindrical curvature of
the cornea. Thus, a change in the model’s cornea would perhaps be the logical way of
producing this effect. However, this is impractical since the cornea of the model is a fixed lens.
However, the same effect can be accomplished by inserting an additional lens that is "out of
round."
•
•
•
Return to using your "normal" eye model from Activity #3.
Put the object box at 35 cm. Insert a cylindrical concave lens (-5.5 diopters) immediately
behind the cornea, producing astigmatism.
Remember – the crystalline lens you found for near vision in the first activity should still be
in place.
Turn the cylindrical lens a little to make only one line of the image sharp.

Make a sketch of the blurred image and record the lenses that you are using.
•
Becoming an optometrist for an eye model…
Your eye model no longer represents normal vision, but vision with astigmatism.
Assume you are an optometrist and need to prescribe a corrective lens (i.e., glasses) to correct
this patient’s vision.

•
14.
Place in front of the cornea the correcting convex cylindrical lens (1.75 diopters) and turn it
until the image is again sharp.
Change the angle of the rear lens and repeat.
Explain how you think one lens is able to correct this vision defect. Note, the axis of a
cylindrical lens is defined as the line along the thinnest part of the lens.
Activity #6: Modeling Compound Defects with the Eye Model
Astigmatism is often accompanied by farsightedness or nearsightedness You will now
model these compound defects as well as attempt to correct for them.
•
•
•
Be sure to finish Activity #4 before starting Activity #6!
In order to study this phenomenon, place a concave cylindrical lens (-5.50 diopters) at G1
immediately behind the cornea with its cylindrical axis vertical.
In addition, place the retina in the position to give myopia. Make a note of how you model
this eye defect.
Becoming an optometrist for an eye model…
Assume you are an optometrist and need to prescribe corrective lenses (i.e., glasses) to
correct this eye's vision.
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

The Amazing Human Eye
Correct the eye’s vision by choosing the proper combination of eyeglass lenses (S1 and S2).
Record the kinds of lenses used and the results of the lens combination.
In actual practice the two correcting lenses are combined into a single eyeglass lens.
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The Amazing Human Eye
Activity #7: Modeling the Treatment of a Cataract with the Eye Model
In the eye disease known as a cataract, the crystalline lens becomes opaque. When this
condition exists, the crystalline lens is often removed.
•
•
15.
Return the eye model so that it represents a "normal" eye.
Remove the lens L from your model (modeling cataract surgery)
(a)
Is vision still possible for someone who has a lens removed? Explain.
(b)
What do you need for the eye to see clearly? With this eyeglass lens, at what
distance(s) is the image still distinct? Would another lens allow vision at another
distance? Test your hypothesis and describe the results.
Activity #8: Applying What You Learned to Your Own Eyes
16.
(a)
Using what you know about image formation with lenses and with this eye model,
do you think the optic nerve in your eyes is located at the center of retina, between
the center and your nose, or between the center and your ear? Explain the reasoning
for your choice, using sketches when helpful.
Hint! Think about the results of your blind spot test!
(b)
Based on your reasoning, is your eye model a human right eye or left eye? Explain
and draw a sketch of your evidence.
Activity #9: Wrap Up
17.
Describe in your own words at least two ways that the eye model is a good model for
the behavior of the human eye and at least two ways that the eye model is not a good
model for the behavior of the human eye.
Please do the following before you leave…
•
•
•
•
•
At the conclusion of this experiment, be sure there are no lenses left in the model.
Empty and rinse the eye model at the sink in the hallway and dry it with paper towel.
Be sure to also dry each lens and put them back into their small metal box.
You should only have one of each type of lens and aperture at your lab station.
Be sure all "patients" have been returned to your instructor.
Thank you!
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*
*
The Amazing Human Eye
This write-up was largely taken from I. Bassett, et al. General Physics Laboratory Experiments, Brigham Young
University, Provo, Utah.
Additional information and graphics were taken from H. Q. Fuller, et al. Physics Including Human Applications,
Harper & Row, New York, 1978.
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