Download optics of the human eye lab

Optics of the Human Eye
Physiology 2
4/17
Name___________________________ Per._________
A. Introduction
The human eye is a complex organ made of several systems and structures functioning together to provide
us with our sense of sight. As light reflects off of an object such as a tree, some of the light waves travel to
the eye. Waves of light enter the eye through the pupil. The iris opens and closes, controlling the size of the
pupil and thereby the amount of light that enters the eye. The cornea and the lens, two structures made of
transparent tissue, make up the optics system of the eye which is designed to focus the light entering the
eye on the retina by bending, or refracting it. Extrinsic muscles surrounding the eye change its shape and
that of the lenses to focus it, similar to the way a telescope works. In a normal eye, an image forms on the
retina. Rods and cones on the retina process light into a nerve impulse, which is carried by the optic nerve
to the brain. The brain interprets the impulse into the images we perceive as sight.
Sometimes, small deformities in the eye can interfere with its ability to focus light correctly on the retina.
The eye of a person who is farsighted (hyperopia) is slightly shorter than normal, and the lenses too curved.
The focal point is located behind the retina, and the person self-corrects by keeping the object at a greater
distance from the eye (“far-sightedness”). The eye of a person who is nearsighted (myopia) is slightly longer
than normal, and the lenses not curved enough. The focal point is located within the posterior cavity of the
eye, and the person self-corrects by keeping the object closer to the eye (“near-sightedness”).
In this activity, you’ll use two models showing the structures included in the optics system of the normal
human eye to understand how they allow the formation of an image. The first will be a physical model and
you’ll build it using an optics bench, the second will be a conceptual model called a ray diagram.
In the extension, you’ll build a physical and conceptual model of an abnormal human eye affected by a
common condition (myopia or hyperopia) and correct its vision.
B. Normal Eye: Physical Model
1. Eye Model Design
Build a model of the human eye using the following parts. Indicate below what structure of the eye they
represent:
Model Part
Optics bench
Human Eye Structure/s
Light source
Viewing screen
Convex lenses (+mm)
Place the light source on the optics bench at the 0 cm mark and turn it on. Place the viewing screen across
from it. Use the convex lenses to create a model of an eye “seeing” an image identical (in size and focus) to
the one projected by the light source.
How many convex lenses di you use? Explain.
How did you have to adjust your model in order for the eye to “see”?
What did you observe about the image focused on the viewing screen? How does that compare to the
image focused on the retina?
Draw a sketch of your physical model and record the position in cm of each part.
2. Increased Distance Simulation
Increase the distance between the light source and the “eye” (each component) by 10 cm.
Can your eye still “see” clearly? If not, what do you need to do in order for the image on the viewing screen
to be in focus?
Use your conclusions to explain how a normal eye adjusts to seeing objects at different distances.
C. Normal Eye: Conceptual Model
Geometric optics explains how optical devices (such as lenses and mirrors) create images by considering
that light travels in waves, which can be conceptualized as straight lines emanating from a light source or
reflected from an object. A ray diagram is a useful tool in geometric optics because it describes and predicts
images formed by lenses or mirrors. It can help designing microscopes, telescopes, movie projectors, and
corrective eye glasses and contact lenses.
Consider Ray Diagram 1 in the next page, which is for a double convex lens. The left arrow represents an
object such as a tree. Rays traveling parallel to the center axis (perpendicular to the center of the lens) refract
so that they pass through the focal point on the other side of the lens. The place where the rays converge
shows where the top of the arrow would be in the image formed by the lens.
Ray Diagram 2 shows how the eye’s double convex lens forms an image on the retina. Study these
diagrams. You’ll modify them to understand how myopia and hyperopia affect vision, and how they can be
corrected.
Ray Diagram 1: Double Convex Lens
Ray Diagram 2: Normal Human Eye
D. Abnormal Eye: Physical Model
Choose one eye condition (myopia or hyperopia) and record it here _______________________________.
Place the parts of the model back to the locations (cm) recorded in your sketch. Use the information in the
Introduction to modify the model to simulate the anatomy of an eye affected by the condition you chose
(the image on the viewing screen will be out of focus).
Draw a sketch of your physical model and record the new position in cm of each part.
Explain how you modified your model in order to simulate your condition and why the eye sees out of
focus.
How can you move the light source in order to focus the object? Explain.
Which of the two additional lenses (+mm is convex and –mm is concave) can you use to correct your
condition? Research convex and concave lenses and explain why the type of lens you choose corrected your
condition.
Use your results to predict the anatomy of the opposite condition, how an object could be moved to correct
it, and what type of lenses can be used to correct it.
E. Abnormal Eye: Conceptual Model
In this part of the lab, you’ll apply the information you learned in part D about the anatomy of an abnormal
eye, and how it affects vision, to draw a ray diagram of an eye affected by myopia and one affected by
hyperopia.
1. Modify the ray diagram of the eye in Part C of this lab to simulate both conditions.
Ray Diagram 3: Human Eye with Myopia
Ray Diagram 4: Human Eye with Hyperopia
2. Redraw each diagram above by adding the appropriate corrective lens (convex or concave).
Ray Diagram 5: Human Eye with Corrected
Myopia
Ray Diagram 6: Human Eye with Corrected
Hyperopia