LEDs are generally fed with DC, as current only flows in one direction, but they flicker as the power coming into them fluctuates -- the more ripple, the more f

LEDs are generally fed with DC, as current only flows in one direction, but they flicker as the power coming into them fluctuates -- the more ripple, the more flicker. Cheap LED holiday lights don't even bother rectifying the AC voltage across the lights, so they are only on for half the cycle, giving them a very noticeable flicker. It's especially apparent as you scan your eyes past them. In contrast, incandescents allow current to flow both directions, and will glow for a little while after current stops flowing, so they don't appear to flicker on 60/50hz mains current.