Materials
Small steel can
Large round balloon
Tape (duct tape or electrical tape
is best)
Sticky putty
Small (0.5cm) plastic mirror
Laser pointer
Corner molding, 25cm
Rubber bands
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Cheapie Oscilloscope
from Al Guenther's Science Solutions
Background: The Light Lever
Many scientific measuring instruments have dials with pointers to
indicate the magnitude of the measurements. A pointer is actually a
lever which magnifies the distance that the instrument moves. In ordinary clocks, speedometers and electric meters
for
example, the
rotating shafts move very small distances, but the pointers greatly
magnify these motions so they are easy to see. Unfortunately,
pointers have mass which the instrument must move. In very sensitive
instruments this presents a problem, as the force which the
instrument measures is too weak to overcome the mass of the pointer.
The solution to the problem is a massless pointer--a beam of light
referred to as a light lever! A small mirror is attached to the
instrument. A beam of light is reflected off the mirror onto a
screen which may be several meters distant. The beam is, in effect,
a massless lever several meters long which greatly magnifies a very
small movement of the instrument.
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Construction
Remove both ends of a small steel can such as a soup, nut or single
serving juice can. Cut a large round balloon from the mouth, over
the top and back down to the mouth. Stretch one of the two halves
over one end of the can and tape it securely (see diagram). For
additional security it is wise to also wrap tape around the side of
the can over the rubber and tape strips.
Use sticky putty (available at stationery stores for temporarily
hanging posters) to fasten a small (0.5cm) mirror to the rubber
midway between the center and the edge. A plastic mirror is best.
The plastic mirror can be cut with a hacksaw or broken into small pieces
with pliers (WEAR GOGGLES).
For a more effective oscilloscope cover the end opposite the rubber
with a snap-on plastic lid. Cut a hole about 2cm in diameter in the
center of the lid.
The laser and the can assembly must be attached to a holder to
maintain proper alignment. To make the holder, obtain a piece of
corner molding about 25cm long (from a lumber yard). Insert a screw
about 10cm from one end (see diagram). Adjust the screw so that the
laser beam strikes the mirror and reflects onto a wall.
When the laser and can assembly are properly adjusted, fasten them
to the molding with rubber bands or tape. CAUTION: TAKE GREAT CARE
TO ASSURE THAT NEITHER THE LASER BEAM NOR ITS MIRROR REFLECTION
STRIKES ANYONE'S EYES.
Procedure:
With your mouth close to the open end of the can, make a variety of
sounds of varying pitch, ranging from musical notes to raucous
noise. You should observe a variety of patterns formed by the laser
beam (the light lever) as it reflects from the vibrating mirror onto
the shaded surface. CAUTION STUDENTS NOT TO LOOK DIRECTLY INTO THE
LASER.
Things to try and notice: How is the pattern different for musical
notes and nonmusical noise? Who can make the most regular geometric
pattern? How does the distance from the wall affect the pattern? Try
different materials in place of the balloon (like plastic wrap).
What happens if objects are in the can? Try different sound sources
like musical instruments.
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