So my phone has a little flash-light that flashes (square wave) at 120 ± 2 Hz. It's easy to see if you move it around quickly, you get a trace like an oscilloscope would show. However there's a certain speed where if you move it slowly enough, all of a sudden your eye starts to track it properly and you can't tell that it's flashing any more.
This would be a good way to cheaply determine eye-response.
Just saying.
