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Activity
8. Photographing waveforms on an elastic band
Equipment needed:
- Vivitar 283 flash unit
- 4 AA batteries or SB-4 AC adapter
- Tape recorder sound trigger
- SLR camera and film
- Tripod
- Large rubber band or elastic cord
Suppose an elastic cord is stretched end-from-end, pulled to the
side and released. What shape will it take? The
answer is surprising to most people. If you want to find out
the answer for yourself, don't click
here. Instead, read on.
With a sensitive sound trigger, a snapped elastic cord can be
observed almost immediately after its release. A sound trigger made
from a tape recorder works well. Go
here for instructions on how to construct the trigger.
Use a large rubber band or a length of elastic cord. If you use a
rubber band, cut it so that you can stretch it end-to-end. Hold the
microphone of the tape recorder near the point where the band will
be plucked. Aim the flash unit at the band. When the band is plucked
and the flash discharges, the initial waveform can be observed. Move
the microphone farther from the release point to observe the
waveform at a later time.
| Tip: When
photographing small subjects against a dark background, the
flash unit's automatic-exposure circuit is influenced more
by the light reflected from the background than from the
subject. When the background is dark and far away, the
intensity of the reflected light at the flash unit's
photocell will be low, causing the flash duration to be
larger than desired. The problem can be reduced by
hanging a background (even a dark one) within about a meter
of the subject. Alternatively, one could control flash
duration using other methods such as those described in Activity
2. |
| Tip: A problem
in taking photographs with a sensitive sound trigger is that
the click of the camera shutter may actuate the trigger
prematurely, giving unwanted exposures. To avoid this, hold
a hand over the camera lens while opening the shutter. |
The next activity will introduce you to another trigger that can
be used to capture the plucked cord and many other phenomena.
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