Vision

Objective:

To explain in short essays or diagrams why the focal distances of emmetropic, myopic, and hyperopic eyes will be different and how to compensate for these differences with corrective lenses, at the level of 85% proficiency for each student.

In order to achieve this objective, you will need to be able to:

Measure focal distances in an artificial eye model.
Measure focal distances in emmetropic, myopic, and hyperopic eyes.
Measure the effects of corrective lens on focal distances in emmetropic, myopic, and hyperopic eyes.
To examine the effects of astigmatism, lens removal and pupil size on the vision.

Materials:

Group Supplies:

Artificial Eye Model

Light source for the Artificial Eye Model

Set of Lenses for the Artificial Eye Model

1 Meter Stick

Desk lamp

Methods and Results:

INSTRUCTIONS FOR THE USE OF THE ARTIFICIAL EYE MODEL APPARATUS

The model eye used in this experiment is a metal container shaped roughly like a horizontal section of the eyeball. A window in one side of the tank is covered with a meniscus lens C which, serves as the cornea. The tank is filled with water, which takes the place of the aqueous and vitreous humors. The interchangeable crystalline lens L is mounted in a septum, which marks the boundary between the humors. In front of the cornea are two additional supports S1 and S2 for spectacle lenses. Two supports G1 and G2 are provided for the insertion of additional lenses and a diaphragm. The retina is represented by a circular white area on a removable curved screen R, which may be located at various positions by means of a series of groves in the well of the tank. The blind spot is represented by a back spot painted on the retina.

A diaphragm and the following set of six lenses are mounted individually in metal holders.

1. Spherical convergent (+7.00 d)

2. Spherical convergent (+20.00 d)

3. Spherical convergent (+2.00 d)

4. Spherical divergent (-1.75 d)

5. Cylindrical divergent (-5.50 d)

6. Cylindrical convergent (+1.75 d)

In the case of the cylindrical lenses the axis of the cylinder is indicated on the mount. An illuminated object box with a suitable geometric design for demonstrating visual defects completes the equipment.

Description provided by manufacturer

Vision in the Emmetropic, Myopic and Hyperopic Eye

Fill the eye tank with water just above the flange at the cornea. Experiment with the model in each of the ten configurations shown in Table 1.

Table 1. Configurations of the Eye Model:

Emmetropic

(retina in middle slot)

Myopic

(retina in posterior slot)

Hyperopic

(retina in anterior slot)

unaccommodated

(+7 lens in position L)

accommodated

(+20 lens in position L)

unaccommodated

(+7 lens in position L)

corrected

use –1.75 lens

in S₁ position

use +2.00 lens

in S₁ position

accommodated

(+20 lens in position L)

corrected

use –1.75 lens

in S₁ position

use +2.00 lens

in S₁ position

The “S1” and “S2” positions are in front of the artificial cornea. The “G1” and “G2” positions are just behind the artificial cornea and correspond to the region containing the aqueous humor. The “G1” position corresponds to the location of the iris. The “L” position is behind “G1” and “G2” and corresponds to the location of the lens.

A + diopter lens is convex and a – diopter lens is concave.

Emmetropic refers to a normal length eye. Myopic refers to an elongated eye where the retina is too far from the front of the eye. Hyperopic refers to a shortened eye where the retina is too close to the front of the eye.

Unaccommodated refers to the lens being adjusted for distant vision. Accommodated refers to the lens being adjusted for close vision.

For each configuration (1-10) answer the following:

a) Are objects greater than 20 feet away in focus? Why or why not? Prepare a diagram illustrating the path of light passing from the object through the eye lens system to the point of focus. Record your results in Table 2.

b) What is the closest distance that objects are in focus? Prepare a diagram illustrating the path of light passing from the object through the eye lens system to the point of focus. Record your results in Table 2.

Table 2. Observations using the eye model:

Emmetropic

(retina in middle slot)

Myopic

(retina in posterior slot)

Hyperopic

(retina in anterior slot)

unaccommodated

(+7 lens in position L)

In focus at 20 feet or more?

______

Near Focus Distance is

_____

In focus at 20 feet or more?

______

Near Focus Distance is

_____

In focus at 20 feet or more?

______

Near Focus Distance is

_____

accommodated

(+20 lens in position L)

In focus at 20 feet or more?

______

Near Focus Distance is

_____

In focus at 20 feet or more?

______

Near Focus Distance is

_____

In focus at 20 feet or more?

______

Near Focus Distance is

_____

Table 2. Observations using the eye model (continued):

Emmetropic

(retina in middle slot)

Myopic

(retina in posterior slot)

Hyperopic

(retina in anterior slot)

unaccommodated

(+7 lens in position L)

corrected

use –1.75 lens

in S₁ position

In focus at 20 feet or more?

______

Near Focus Distance is

_____

use +2.00 lens

in S₁ position

In focus at 20 feet or more?

______

Near Focus Distance is

_____

accommodated

(+20 lens in position L)

corrected

use –1.75 lens

in S₁ position

In focus at 20 feet or more?

______

Near Focus Distance is

_____

use +2.00 lens

in S₁ position

In focus at 20 feet or more?

______

Near Focus Distance is

_____

Astigmatism:

Place the retina of the eye model in the normal position (R). Put the lens (+20.00) in the crystalline lens position (L) and the lens (-5.50) in slot G1 (between the lens and the cornea). The axis of rotation of this lens should be vertical. This combination of lenses creates at eye with an astigmatism.

Place the light source the same distance from the cornea as was observed for he Near Point of Vision for the Emmetropic Accommodated Eye. The distance is ______________ cm.

Describe the appearance of the image when an eye has an astigmatism.

Place a lens (+1.75) in slot S1 as a corrective lens and rotate this lens until all lines of the object are clearly focused on the retina. Describe the axis of orientation of the spectacle lens (+1.75) with respect to the axis of orientation of the astigmatic lens (-5.50). Note the surface of the astigmatic lens.

Lens Removal:

Sometimes the lens must be removed due to a cataract as an example. Using the eye model, remove the lens from position (L) and place lens (+7.00) in the S1 or spectacle slot. Note that vision is still possible with corrective lenses even though the lens is absent. Record your observations. What are the visual restrictions in this case?

Pupil Size:

Place the retina in the normal position (R), the lens (+20.00) in the (L) position, and adjust the light source distance such that the image is clearly focused on the retina. Insert the black diaphragm into slot G1 behind the cornea. What is the effect of closing down the pupil size on the brightness and sharpness of the image? Try moving the light source closer and farther away from the eye with and without the diaphragm. In which situation is the depth of focus greater? Record your observations.

Discussion:

Describe the retinal images observed in this exercise. Speculate on three ways humans estimate the size of objects in their visual field.
Describe the visual performance of the emmetropic, myopic, and hyperopic eye. How close and how far can each eye "see"?
What is accommodation? Describe how accommodation occurs. What muscles are involved? What are the changes in the pupil, the lens, and the position of the eyeballs as an object moves closer to the viewer? Explain these changes.
What is an astigmatism?
What is the blind spot? Why is it called a blind spot? Why do we usually not notice the effects of the blind spot?
What is presbyopia?
Why does squinting help? Cite data from the eye model experiment to support your answer.
Why are people with hyperopia more likely to get an eye strain headache?
Why is a severe astigmatism more difficult to correct with contact lenses?