Mechanism of Vision

Celestine Miller Jeffreys Beethoven School
25 West 47th Street
Chicago IL 60609


This lesson is designed for junior high school students. The objectives are:
learning the way light reflected from objects enters the eye; learning the basic
anatomy and physiology of the eye; and learning the basic mechanism of

Materials Needed:

For 5 groups of students (4-5 in each group) The Device 10 cans of equal size (soup, tomato paste, etc.) with the lids on both ends cut out 5 holepunchers 5 toothpicks 15 rubberbands OR tape for each group large (18"x24") black arrow aluminum foil wax paper Perception Tools 5 sets of the following postcards: --1 classic European painting, painted between 1500 and 1800 --1 photograph, preferably 20th century --1 "primitive," American folkart or European painting before 1400 5-8 optical illusions (not the Magic Eye) Strategy:

1. Have each group cut 2 generously-sized squares of aluminum foil and one
generously-sized square of wax paper. The square should fit over the ends of
the cans.
2. In one aluminum foil square, use the hole puncher to punch a hole
in the center of the square (square A). In the other aluminum foil square, use
the toothpick to punch a very small hole in the center of the square (square C).
Do nothing to the wax paper.
3. Secure each square to the can with tape or rubber band.
4. Cover one end of a can with square A. Do nothing to the other end of that
can. Cover one end of the other can with square B and the opposite end with
square C. See diagram below.

`fffffffffffffffffffffp `fffffffffffffffffffffp
w w w w
w w w w
w w w w
afffffffffffffffffffffq afffffffffffffffffffffq
| | | |
square C square B nothing square A

5. Before you begin the next phase, be sure to have the arrow taped in an
upright position to a very bright window. Hold the device in the manner
illustrated above, with square B touching the open end of the first can. Look
at the arrow. Ask the following questions:

a. What do you see?
b. How did you have to arrange the device in order to get the best image?
c. Why does the image look that way?

Explanation: light is reflected from an object and enters the lens of the eye,
where it is refracted and travels to a group of light-sensitive cells in the
back of the eye called the retina. Light entering at the top of the lens is
refracted to the bottom of the retina; light entering the bottom of the lens is
refracted to the top of the retina; light entering the center of the lens is
refracted to the center of the retina. Under normal conditions, light reflected
from an object is projected onto the back of the retina so that the object
projected is upside down and switched from left to right. We don't perceive
objects in this manner; the visual cortex "corrects" this so we see the object
in its correct orientation. The device strains the light coming to the lens at
the top and bottom, so that your perception of the object is fooled into seeing
it upside down.


1. To use the perception tools, have each student study the three pictures.
(Note: you must use the pictures described in the materials section. European
paintings before 1400, as well as "primitive" art, do not have depth of
perception. You also must be careful with Impressionist art because it has a
tendency to be very "flat.") Ask the following questions:

a. What is the subject of the painting?
b. List some objects in the foreground and background.
c. In what ways can you differentiate between the objects in the
foreground and the background?
d. How did the artist conceptualize foreground and background objects?
e. Do all three pictures have depth?
f. Draw a three-dimensional picture using the techniques you've observed.

2. To use the optical illusions, tape each one on the blackboard or around the
room. Have each student look at the illusion for only 30 seconds or so. Ask
them to quickly write down what they've seen. It's important that students do
not talk or discuss their answers until everyone has looked at the pictures.
Discuss the answers with your class. Ask them what lines or contours caused
them to see the drawing as they did.

Explanation: light is reflected from objects, projected onto the retina and
perceived in the visual cortex. Light is projected onto the retina as a two
dimensional representation of three dimensional space. Depth perception takes
place because our eyes are not in the same position and we perceive depth by
the space between the different objects as they are projected onto the retina.
Part of depth perception concerns our experience with the world. We touch
objects as infants and can determine if the object is near or far. These
experiences are imbedded in our visual cortex and we rely on them when we
perceive the depth and distance of new objects. When an artist represents three
dimensional space in two dimensions, she uses the same references that our
brains use such as the size of an object (the further away, the smaller it is);
the clarity of an object (the further away, the less distinct it is); lines of
depth (straight lines going from near objects to far objects). Furthermore,
contours of objects are perceived in the visual cortex. Cells in the cortex are
more stimulated by changes in contours than by flat, continuous space.

Performance Assessment:

Each phase of the demonstration should be accompanied by a written lab report.
Students will write their answers on this sheet. For an excellent grade
concerning the device, students should: assemble the device in the manner
illustrated; view the arrow upside down; explain that light is constricted by
the device and causes the image to appear upside down. In assessing the
appropriateness of the device, the instructor should observe that the holes are
punched in the correct places, all squares are securely fastened to the cans,
the device held in the correct manner, the device used to see the arrow in the
correct manner. Concerning the three pictures, students should: accurately list
foreground and background objects and explain that near objects are larger and
more distinct than distant objects. Concerning the optical illusions students
should explain why they saw the illusion in a certain way and explain that
contours are very important to perception.


Vision is an interplay between light reflected from the object and the mode of
perception in the brain.
Return to Biology Index