Hello, I made a device that vibrates a steel rod. Down the center (horizontally) as it vibrates I can see what appears to be some sort of sine wave of some sort. Any idea what this could be?

https://youtube.com/shorts/YKozQOmhq1o?si=jFK0vLfuaUmwJDF7

 

It's an aliasing effect, mostly likely either due to the video frame rate or the frequency of the lights (or a combination of the two).

 

you can waggle a pencil between your fingers and persistance of vision makes it all wobbly as well

 

The dark sine wave stripe is as @WBahn mentioned, the result of the lighting’s interaction with the shuttter of the camera. you can test this with a non-dimmable* flashlight, which, if you point it at the rod, should swamp the room lighting and make the stripe disappear. You can also change it’s wavelength by adjusting the frequency at which the rod vibrates. You can test this by adding a very small weight to the end of the rod (even just a few layers of tape) and comparing.

The sine wave appears because for a very short time the room is dark. Persistence of vision, a feature of your eyesight, prevents you seeing this directly. Your eyes “fade out” while the camera turns on and off. Motion pictures depend on persistence of vision (and some other things) to turn a series of motion blurred photos into what looks to us like sharp, smooth action.

In the early days of motion film, frame rates were as low as 15fps. Standard cinematic film is shot at 24fps. This is quite slow and so the early westerns were plagued by the wagonwheel effect, where the spoke wheels on wagons seemed to rotate backwards sometimes.

Doc Edgerton, the stroboscope pioneer who made high speed photography a practical tool, donated an exhibit about stroboscopic light to the Boston Museum of Science whicih showed a controlled stream of water drops apparently frozen in the air or, by adjusting the flash rate, the visitor could make the stream appear to move slowly down, or up.

Dr. Edgerton provided replacement flash tubes for the EG&G stroboscopes his company made and were in the exhibit (The E is for Edgerton). I had the privilege of speaking with him several times to discuss the exhibit or “order” new flash tubes. He was a very pleasant and personable man and for me, who was always in awe of his famous photos of bullets in flight, a great opportunity.

*If you use a dimmable flashlight that uses PWM it may introduce it’s own alias.