Are lightbulb "dimmers" perhaps a little unsafe? ?

I read, once upon a time, that they don't really dim the bulb but rather they cause extremely fast flashes that give the illusion of a dim bulb.  When we look at them, it appears to be a dim bulb and thus safe.  But are we in reality getting lots of extremely bright flashes and therefore even though it appears safe (because of the illusion of dimness) it really isn't?

Also, how do I change the category of this question.  It put it in something like Jokes and Riddles, so I deleted the question.  However, now as I type this, I have a Submit button but no obvious way to change the category.