Spy Physics: Using a Laser to 'hear' a conversation

Document Type


Presentation Date

Winter 1-5-2014

Conference Name

American Association of Physics Teachers Winter Meeting 2014

Conference Location

Orlando, FL


We will present a simply constructed laser experiment for students beyond their first year in physics. The goal of this particular experiment is to not only give students experience with laser interferometry, but to allow students to have greater excitement than they would normally get counting fringes or making precision measurements. Our experiment started when a student asked “Is it true that you could use a laser beam to detect a conversation in an office by its reflection off of a window.” To accomplish this task we used a simple Michelson interferometer in which one mirror vibrated in response to a sound source. A speaker connected to a radio inside a small box with a tinted glass window served as a mirror for the laser beam and also represented the office that we were “spying upon.” At present time, we have tried the experiment in two different ways. One in which the sound source is directly connected to the mirror (the easy version) and one in which the sound must propagate through air to vibrate the mirror (the harder version). We will demonstrate our results if the audio equipment is willing.



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