If you're used to noticing the dots-and-dashes from flicking your eyes past a 60hz (non-incandescent, say LED) light (each about 17ns apart), - something that h

If you're used to noticing the dots-and-dashes from flicking your eyes past a 60hz (non-incandescent, say LED) light (each about 17ns apart), - something that happens to most of us many times per day - 40 vs 60 is a pretty obvious difference, and 17ns an obvious measure for comparison.