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OBJECTIVE
These
activities will offer:
-
a hands-on experience in constructing a stroboscopic viewer
- an
opportunity to view motion that appears stopped due to stroboscopic
effects
- an
understanding of how a gated view may produce the illusion
of "frozen" motion
MATERIALS
- Heavy
stock paper
- Pencil
with an eraser
- Push
pin
-
Flashlight (battery powered)
- Access
to a sink and cold water faucet.
PROCEDURE
Part 1 - CONSTRUCTING THE STROBOSCOPIC DISK
- Trace or copy the template shown here onto a sheet of
heavy stock paper.CLICK
HERE FOR TEMPLATE
- Use scissors to carefully cut out the outline of this
disk.
- Carefully cut out each of the slots that are positioned
along the rim of the disk.
- Insert a pushpin into the center of the disk. Anchor the
pin into the eraser of a pencil. (SE
- Twirl the disk to insure that it spins freely. If it binds
up, move the pin around so that the hole widens.
Template
for the strobe disk
Part
2 - FREEZING THE DRIPPING DROP
PROCEDURE
- Work in a group of two students. Make sure that there
are no AC-electrical devices near the sink.
- Identify the cold water faucet. Open and adjust the faucet
so that a steady stream of large droplets falls into the
sink.
- At this point, you may wish to dim the room. Aim the beam
of the flashlight at the droplets. At the best illumination
angle, the drops will be illuminated and separated from
the sink basin.
- While your partner adjusts the flashlight beam, practice
spinning the slotted disk with a constant and continual
motion. Once a uniform speed is maintained, closes one eye
and peers through the rotating slotted rim. As each slot
rotates into view, it opens a quick "window" through which
to see.
- Once the technique is mastered, the water droplets are
viewed through the spinning slots. In order to FREEZE the
droplet stream, you must spin the disk at a speed that is
in sync with the falling droplets. At the correct speed,
the slotted view will show the next drop in the same region
of space as the previous drop.
- If the drops don't appear stopped in midair, don't give
up. In order to perceive this illusion, the timing between
the falling dorps and the rotating disk must be perfect.
If this gating is not in sync, then your observation won't
generate the illusion. Try changing the rate of the drip
and/or the rotational speed of the disk until you observe
the frozen droplets.
- Exchange roles.
QUESTIONS
- Why did you illuminate the drops in the flashlight beam?
- Why was the rotation speed critical to freezing the droplet
stream?
- Suppose you rotated the disk too quickly. What would you
see?
- Suppose you rotated the disk too slowly. What would you
see?
- What shape did the "frozen" water drops appear to be?
EXTENSIONS
Stop
The Screen
The stroboscopic disk you created can by used to examine all
sorts of phenomena. Did you realize that a monitor screen
continually "redraws" its image? Obviously this happens at
a rate too fast for us to detect. However, if you view the
screen through the rotating slots of your view, you'll be
able to view this refreshing process. At the right speed,
you'll isolate and observe dark bands that extend across the
monitor.
On
Your Own
Find
out how a strobe is used to "time" an automobile engine. If
available, bring one of these engine strobe guns to class
and discuss its operation.
WEB
CONNECTION
University
of Chicago
Read more about Jaeger's and Nagel's research at the University
of Chicago
Strobe
Photo Information
A biography of strobe photography pioneer Harold Doc Edgarton
with strobe photo references
Scientists
Discuss Water Droplets
Communication between scientists on the nature of water droplet
size
The
activities in this guide were contributed by Michael DiSpezio,
a Massachusetts-based science writer and author of "Critical
Thinking Puzzles" and "Awesome Experiments in Light & Sound"
(Sterling Publishing Co., NY).
Academic
Advisors for this guide
Corrine Lowen, Science Department, Wayland Public Schools,
Wayland, MA
Suzanne Panico, Science Department, Fenway High School, Boston,
MA
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