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grade level
subjects
duration
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Two Eyes Are Better Than One
K-th, Supporting PE -LS-,
Life Sciences, Constructing Explanations
Prep time: minutes; Activity time: minutes
Classroom
You can see this eect by holding a pencil out at arm’s length.
Look at the pencil with your right eye closed and your left eye
open. Then switch eyes. It will appear to you that the pencil
has changed position in relation to the background. This
eect, called parallax, occurs because each eye is viewing the
pencil from a slightly dierent perspective. In many animals,
such as humans, both eyes are facing forward and are fairly
close together, so the dierence in perspectives is small. Even
so, the dierence is enough to provide the brain with some
valuable information.
MATERIALS
scratch paper
pencils (1 per student)
SCIENTIFIC TERMS FOR STUDENTS
» parallax: the difference in the apparent relative position of
objects when they are viewed from different perspectives.
» depth perception: the ability to visually judge distances.
BACKGROUND FOR EDUCATORS
Here’s a question you may not have given much thought to
before: Why do humans (and many other animals) have two eyes?
If you close one eye and take a look around, you should nd
that you can still see everything pretty well. One eye can see
with adequate detail, color, and clarity—so what is the point
of having the second eye? While it seems that we should be
able to get along just ne with only one eye, you will be hard-
pressed to nd any one-eyed vertebrates. As it turns out, there
are some interesting advantages to having two eyes.
Parallax
The advantage of having two eyes is all about location. Our
two eyes are located in dierent places, and as a result have
dierent perspectives on the world around us.
OBJECTIVES
Students will be able to:
1. experience how parallax contributes to depth
perception.
2. explain how the phenomenon of parallax is the result
of having two eyes.
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Two Eyes Are Better Than One
animals (including humans) to have two eyes instead of
one.
. Have students cover one eye and ask if they are still able
to see well. Have them brainstorm reasons that two eyes
might be an advantage over having just one eye.
. Tell students that they will learn some of the reasons that
two eyes might be useful through these activities.
PROCEDURE
Part : There’s a hole in my hand!
. Give each student a piece of scratch paper and have him
or her to roll it up into a tube-shape.
. Have students take the tube in their right hand, hold it up
to their right eye, and look through it, as if they are looking
through a telescope.
. Now tell students to place their left hand next to the tube
about two inches from the far end. Their left hand palm
should be facing toward the student’s face and the edge
of the hand should be touching the side of the paper tube.
See the next page for a photo of what this looks like.
. Have students keep both eyes open. Tell them to focus
their eyes on a distant point on the far wall. As students
focus their eyes, they should suddenly see a “hole” appear
in the center of the left palm.
Teacher Tip: Some students may require a little help to get
the illusion to work. Here are some trouble-shooting tips:
» Make sure that the student has both eyes open and
that the hand is not covering the opening of the paper
tube.
» Try moving the left hand slowly closer to the face until
the illusion comes into focus.
» If that doesn’t help, try readjusting where the eyes are
focused. Choose an object on the other side of the
room and tell the student to focus his or her eyes on
the object. If they are still having trouble, have them
choose an object that is closer or farther.
» If none of that works, have the student try switching
eyes. Some people can see the illusion more easily
with one eye than the other.
Depth perception
Take the pencil that you were holding at arms length and hold
it much closer to your face. Then repeat the exercise of looking
with one eye, then the other. You should observe a larger
apparent change in position than when the pencil was farther
away from your face. For objects close to us, there is a big
dierence in the angles between each eye and the object, so
you see the pencil take a big “jump” when you switch eyes. For
objects farther away, the angles are much more similar and the
“jump” when switching eyes is less noticeable.
Our brains use that information to determine how far away
objects are. While we are not consciously aware of the
dierences between the images collected by our left and right
eyes, our brains are subconsciously sorting through those
dierences to calculate how far away each object we see is.
Our depth perception, or ability to judge how far away things
are, depends largely on the fact that we have two eyes viewing
the same objects from slightly dierent perspectives.
This simple activity illustrates the concept of parallax and
serves as a good starting point for discussing depth perception.
Other benets of having two eyes will be discussed in the
“Spot Danger, Track Prey” lesson.
TEACHER PREP
1. Gather scratch paper for each student.
INTRODUCTION
. Begin by asking students why it might be useful for
NGSS CROSSCUTTING CONCEPTS CONNECTION: This
lesson connects to the Crosscutting Concept of Structure
and Function because students are asked to think about
why it might be useful for animals (including humans)
to have two eyes rather than just one. This question
prompts students to consider how a particular structure
(two eyes) might function better than another (having
just one eye). In Parts and , students experience
rsthand why having two eyes located in slightly
dierent places creates the phenomenon of parallax,
which creates depth perception.
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Part : Pencil Parallax
1. Have students hold a pencil out at arms length.
2. Tell students to cover their right eyes and look at the
pencil with their left eyes only. Have them note where
the pencil is in relation to objects in the background.
3. Now tell them to switch eyes (i.e., close the left eye and
look at the pencil with the right eye). Ask them what
they observe. They should notice that the pencil appears
to “jump” from one location to another in relation to
background objects.
4. Discuss with students why the pencil appear to “jump”
when you switch eyes. Since each eye is in a slightly
dierent place, each eye sees a slightly dierent image.
When you switch eyes, you are switching between the two
images, and the subtle dierences become noticeable.
5. Ask students to predict whether the “jump” would be more
noticeable or less noticeable with the pencil held closer
to the face. Have them turn and talk to their neighbor to
compare predictions.
6. Now let students test their prediction by repeating the
exercise with the pencils held very close to their faces.
Ask them what they observe. Does the result t with their
prediction?
7. Students should nd that there is a larger apparent
displacement when the pencil is closer to the face. For
Two Eyes Are Better Than One
objects close to us, there is a big dierence in the angles
between each eye and the object, so you see the pencil
take a big “jump” when you switch eyes. For objects
farther away, the angles are much more similar and the
“jump” when switching eyes is less noticeable. Our brains
use that information to determine how far away objects
are.
WRAPUP
Discuss the following question as a class:
» Why do we have eyes? (While we do not normally notice
parallax, our brains use the phenomenon to figure out how
far away things are. The brain takes in what the right eye
sees and what the left eye sees and combines the images, as
shown by the Hole in Your Hand illusion. At the same time,
the brain evaluates the differences between the two images
and calculates how far away things are. Objects that show
a slight displacement, like the pencil held at arms length,
are interpreted as being far away. Objects that show a big
displacement, like the pencil held up close, are interpreted
as being closer. Our ability to perceive and interpret this
information is called depth perception.)
NGSS CROSSCUTTING CONCEPTS CONNECTION:
This portion of the lesson connects to the Practice of
Constructing Explanations because students are asked to
explain how their two eyes created the optical illusion of
a hole in the palm of their left hand. To do this, students
will have to make one or more claims and support them
with scientic reasoning. Don’t worry if students’ claims
are a bit far-fetched at this point. The next activity will
help them build their understanding of the phenomenon
of parallax.
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Two Eyes Are Better Than One
STANDARDS ADDRESSED
Next Generation Science Standards
Science and Engineering Practices Disciplinary Core Ideas Crosscutting Concepts
Constructing Explanations
K-: Make observations (rst-hand or
from media) to construct an evidence-
based account for natural phenomena.
-: Use evidence (e.g., measurements,
observations, patterns) to construct or
support an explanation.
-: Apply scientic ideas, principles,
and/or evidence to construct, revise,
and/or use an explanation for real-world
phenomena.
.LS.A: Structure and Function
All organisms have external parts.
Dierent animals use their body parts in
dierent ways to see, hear, grasp objects,
protect themselves, move from place to
place, and seek, nd, and take in food,
water and air.
.LS.D: Information Processing
Animals have body parts that capture
and convey dierent kinds of information
needed for growth and survival.
.LS.A: Structure and function
Plants and animals have both internal
and external structures that serve various
functions in growth, survival, behavior,
and reproduction.
.LS.D: Information Processing
Dierent sense receptors are specialized
for particular kinds of information, which
may be then processed by the animal’s
brain.
Structure and Function
K-: Students observe the shape and
stability of structures of natural and
designed objects are related to their
function(s).
-: Students analyze many complex
natural and designed structures and
systems to determine how they function.
Related Performance Expectations
Remember, performance expectations are not a set of instructional or assessment tasks. They are statements of what students
should be able to do after instruction. This activity or unit is just one of many that could help prepare your students to perform the
following hypothetical tasks that demonstrate their understanding:
-LS-. Construct an argument that plants and animals have internal and external structures that function to support
survival, growth, behavior, and reproduction. [Clarication Statement: Examples of structures could include
thorns, stems, roots, colored petals, heart, stomach, lung, brain, and skin.] [Assessment Boundary: Assessment is
limited to macroscopic structures within plant and animal systems.]
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Two Eyes Are Better Than One
-LS-. Use a model to describe that animals receive dierent types of information through their senses, process the
information in their brain, and respond to the information in dierent ways. [Clarication Statement: Emphasis
is on systems of information transfer.] [Assessment Boundary: Assessment does not include the mechanisms
by which the brain stores and recalls information or the mechanisms of how sensory receptors function.]
