I was wondering about that - thanks for the information and the nice animation! I guess I was thinking a diode only lights when the current is going one direction and was confused over the meaning of a cycle.

I suppose there could be a pair of diodes lighting on both sides of the waveform made into one unit. Then maybe the flashing would be 120 times a second from what your saying - with the light only going off when the wave briefly crosses from plus to minus. The new LED bulb replacements don't seem to flicker at all - they are more expensive though.