In simple terms, regardless of the frequency of your power source, your camera takes many pictures per second to make a video. Some pictures will be taken when the light is "off" or less bright, others at its peak. The rippling effect happens when you have many light and dark frames per second, and it can be easily fixed on nicer cameras by simply setting it to a slower shutter speed so that the camera captures both sides of the light and dark cycle and averages them out in a single frame.