Question

My students saw a video where a cell-phone was placed inside an acoustic guitar, camera facing up, and the strings plucked. The strings looked like they were vibrating in wave-shapes, sinusoidal and sawtooth and even more complex ones. Is this real or a phenomenon caused the the recording parameters of the camera-app?

Answer 1

It’s a fake. From the two frets in view, we can see that the whole field of view of the camera is only a few centimeters in the foreground. From this and the movie, we would naively deduce that the largest amplitude waves on the strings had wavelengths of only a centimeter or so. In reality, the waves with these wavelengths will have very small amplitudes, and the large amplitude vibrations will have wavelengths of tens of centimeters.
So there you have it – the video is misleading, the effect comes from the camera work, not from physics.