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Activity #5
How Many Pencils?
Grade Level: K-4
Module: Visual Perception
Overview
The students investigate binocular and single (monocular) vision work. Our
two eyes provide us with two images of everything we see. Our eyes look
at the world from slightly different positions on either side of the nose.
This binocular vision allows us to see things in three dimensions, with
depth perception, rather than as flat, two-dimensional pictures. Binocular
vision helps us to judge distance, size of objects and to appreciate perspective.
When NASA designs helmets for the astronauts to wear, it is extremely
important that their vision is not obstructed or distorted in any way.
Key Questions
- What will happen to a pencil when it is placed behind a glass of water?
- Will something similar happen if the pencil is placed behind a similar
translucent liquid other than water?
- Will wider objects have as dramatic effect as the pencil?
Time Frame:
1 class period
Materials
For each group of four students:
- clear glass or clear plastic cup filled with water
- pencil
- flat surface, table top
For each student*:
How Many Pencils? Worksheet
Name:________________________________Date:________________________________
- Place a glass of water on a table and stand a pencil about 30 cm behind
it.
- Look through the glass towards the pencil.
How many pencils do you see? ____________
- Close your left eye.
Do you see the right pencil or the left one?_________
- Close your right eye.
Do you see the right pencil or the left one? ________
- Why do you think the pencils disappear?
Write your own hypothesis.
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
_____________________________________________
Getting Ready
Gather all the materials listed above.
Familiarize yourself with procedure on the worksheet. The water is working
as a lens to produce the images but because the water is held in a cylinder
shape each eye looks through the water at a slightly different angle.
With both eyes open, you will see two pencils. With one eye open you will
see only one image. If your right eye is open you will see the image to
the left disappear, if the left eye is open you will see the image to
the right disappear.
Classroom Activity
- Divide the class into groups with 4 students per group.
- Hand out materials to the groups and have each group place a glass
of water on a table and stand a pencil about 30 centimeters behind it.
Students can take turns holding the pencil upright.
- Tell students to take turns looking through the glass towards the
pencil. Ask them, "How many pencils do you see through the glass of
water?" For older students, ask them to write answers on their worksheets.
They should see two pencils. Go around and assist students as needed-sometimes
they have trouble positioning themselves right to see the illusion.
Ask, "Are there really two pencils?" (No!)
- Now tell the students, "Close your left eye and look at the pencil
through the cup. You can use a hand to cover your eye." For very young
children, you may need to review which hand is the right hand and which
is the left. You can ask the children to raise their left hands. If
they have difficulty, try standing with your back towards them, raise
your left hand and have them imitate.
- Ask the students, "When you had your left eye closed or covered, how
many pencils did you see?" (Only one.) Explain that before, with both
eyes, they saw two pencils, a right one and a left one. Ask, "When you
closed your left eye, which pencil seemed to disappear, the right one
or the left one?" (The right pencil seemed to disappear.)
- Ask, "If you close your right eye, can you guess which pencil will
seem to disappear-the left one or the right one?" Have students guess
and then make observations to see if they were right. Have older students
write down their observations on the worksheet.
Wrap-up Session
- Review the answers that the students gave to the questions previously:
How many pencils did you see when you looked through the water with
both eyes open?
Which pencil seemed to disappear when you closed your left eye?
Which pencil seemed to disappear when you closed your right eye?
- Have the students try to explain why the pencils seem to disappear.
(Of course, there were never really two pencils in the first place!)
For older students, you may want to explain how pictures from the left
eye go to the right side of the brain and pictures from the right eye
go to the left side of the brain.
- Explain to the children that NASA space scientists design helmets
for the astronauts to wear in space, and it is extremely important that
the astronaut's vision is not blocked in any way. The engineers who
work in this area must be sure that the materials used to make the helmet
shield will not distort the astronaut's field of vision or create optical
illusions. Ask the children what might happen if astronauts started
seeing illusions like what the children just saw with the pencils.
More Activity Ideas
Have the students develop a procedure in which astronauts can train for
the possibility of vision distortion. How would the astronauts be able to
overcome any possible illusion? Would it be possible to design a device
that the astronauts can carry outside of the shuttle to correct their vision
in the case of such an incident.
Background for Teachers
Prerequisite:
- Students must be able to carefully handle water in a glass without
spilling
Vocabulary:
- Binocular vision - seeing with two eyes
- Lens - glass or other clear material with a curved shape that causes
rays of light passing through it to come closer together or spread wider
apart
- Perception - the act of being aware, observant
- Perspective - visualizing objects in a 3-D manner; a visual sense
of depth or relative distance
Skills:
- Recognize the idea that each eye perceives objects differently than
the other and that both must work together to create the full effect.
- Understand the difficulty that a person with vision in only one eye
has in seeing things three dimensionally, with perspective.
Concepts:
- Light refracts (bends) through various surfaces and liquids.
Keyword:Visual Perception
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