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21 December 2010

Mail Time: Waves and sound on the AP physics B exam

Paul Dugas, of Woodbridge, VA, writes:

I was wondering about the waves and sound portion of the AP B curriculum.  The description is very short.  In looking at some past AP questions, there are some detailed sound interference pattern questions and sound intensity as a function of time.  The equations for intensity do not appear on the equation sheet AP gives to students.  Do you teach these concepts and, if so, how much time do you spend on them?

Certainly the double slit diffraction pattern for light, sound, and even subatomic particles has appeared on numerous exams. Students are expected to understand qualitatively how the intensity of a sound or light beam will vary on a screen when the wave is passed through a single slit, double slit, or diffraction grating. For the double slit / diffraction grating, they must be able to make the quantitative prediction of the location of the bright and dark spots.
 
I don't remember what intensity "as a function of time" question there might have been. 
 
The relevant principles here deal with light or sound intensity as a function of space.  And we don't have specific equations for a continuous I(x) function, but we know the locations of the bright and dark spots.  So we can fill in the blanks, knowing that a single or double slit diffraction pattern changes intensity gradually, while a diffraction grating produces sharp bright spots.

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