Camera Flash, Pan, and Ping
Country: United States
Date: Spring 2010
If a good camera flash is placed a few inches from a sheet metal
pan, a surprising audible ping will occur. Is this the result of the
transfer of momentum - or the transfer of energy directly to the metal
atoms/electrons? And is Heat always the intermediary of this 'work', or is
it more of a 'direct' effect of the photons on the metal causing them to
Before invoking a complex mechanisms, you need to eliminate simpler
mechanism(s). Occam's razor is the name applied to this principle.
In your observation, for example, you need to eliminate the possibility that
the flash sets off an atmospheric wave of hot air. You would need to devise
an experimental configuration the eliminates, or significantly reduces, the
effect of a wave of "hot" air by conduction. Putting the system in a modest
vacuum could resolve this issue. At low pressure, the transfer of thermal
energy would be reduced significantly.
My experience (which certainly can be challenged) is that Occam's razor
applies. The "pinging" may be a transfer of momentum in the classical sense.
In any case you have an interesting and challenging project.
The "ping" effect is due to the momentary heating of the surface of
the pan by the intense light.
This is known as the "photoacoustic effect," which dates back to
1880 when Alexander Graham Bell noticed that an interrupted beam of
sunlight could make a thin disk emit sound.
Basically, light is absorbed by the surface ---> causing the surface
to be heated --> causing the local area to expand ---> generating a
pressure wave ---> that makes the pan vibrate ---> that we hear as a "ping".
Photoacoustic spectroscopy is used to study solids, liquids, and
gases. A bright source of light (flash tube or laser) and a sensor
(microphone or piezo element) are used, along with electronic
equipment to make it work. The solid, liquid, or gas basically
"pings" when hit by intense light.
You may ask: "how can a pan warming up a fraction of a degree make
sound?" The answer is that the pan warms up very fast. That is
because the power of a pulse of light from a flash may be tens of
thousands of watts, or more, for a short time (maybe 1/1000
second). Even that little amount of heating and vibration is enough to hear.
If the flash were proportionally slower and dimmer, the same heating
would occur, but there would be no "ping" because our ears are not
good for hearing very faint low sounds.
My flash is not so powerful, so I must place it a centimeter or two
away from the pan for the sound to occur. If the pan is painted
black then the sound is much louder! Light does have momentum, but
it is very small and is not the cause of the ping.
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Update: June 2012