This is a practical demonstration of the moiré principle, an interference effect that appears when two repeating patterns overlap with slightly different spacing or timing, producing a larger, slower pattern that is not physically present in either one. In this clip, the ruler’s edge and fine surface details become a fast moving, repeating pattern as it vibrates. In shadow, the lighting is effectively more directional and higher contrast at the boundary, so small changes along the ruler translate into stronger light, dark modulation. In direct light, the reflections and gradients change, which alters that modulation. Your eye, and especially the camera sensor, then “samples” that fast pattern at discrete intervals (pixels and frame rate). When the vibration frequency, the camera’s sampling, and the lighting contrast are slightly out of sync, you get an emergent low-frequency ripple, the same “beat” phenomenon you see when two close musical notes produce a slow wobble.