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Name: Alex
Status: Student
Grade: 12+
Location: CT
Country: United States
Date: Spring 2010


Question:
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 distort suddenly?



Replies:
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.

Vince Calder


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.

Robert Erck



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