The recent issue of "The Physics Teacher" has an article titled "Hearing
Light." (Phys. Teach, September 2000, page 356.)
We are unable to reproduce an experiment described in this article and we
wonder if anyone else has tried it. The experiment uses a half-blackened
pickle jar to receive the light of an incandescent light bulb. A small hole
in the lid serves as a place for one to place his ear, and the claim is that
the heating/cooling of the air in the jar (as the bulb's light output
fluctuates due to 60-cycle AC current) can be heard as a 120-hertz tone.
Robert Suter, a professor at Bluffton College, first saw this and asked if I
believed it. I did not believe it, so Bob and I tried several experiments,
and we have not been able to detect the claimed effect. Here is what we
tried.
We first tried the experiment as suggested, and heard nothing. Past
experience told us that incandescent light output does not fluctuate much
because the filament cannot cool down very much using 60-Hz AC. We
monitored a 60-watt light bulb with an amplified phototransistor and showed
this is true... the light output does follow a 120-Hz sine curve, but the
amplitude of the fluctuations is not very significant... nothing like the
120-Hz light fluctuations from a florescent tube. Therefore, any thermal
detector would have to be pretty sensitive, and we doubt the pickle jar is
sufficiently sensitive, and perhaps cannot heat/cool fast enough.
To see if a larger effect would produce an audible tone in the pickle jar,
we made a chopper so we could expose the jar to incandescent light that
fluctuated from completely off to completely on. Our phototransistor proved
our chopper was very effective. Again, we could hear nothing at the pickle
jar. The 60-watt bulb was only a couple inches from the jar, with the
chopper between. We even put a piece of rubber tubing from the lid to our
ear (like a stethoscope). If the end of the tube was held near the chopper,
we could easily hear the 100-120-Hz tone that the chopper created in the
air. But when the tube was held to the hole in the pickle-jar lid, we heard
nothing.
Has anyone else tried this? Has anyone else heard anything?
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817