Ive heard that when you flick the light switch on and off repeatedly it damages the bulb. I know this is certainly not true for LED bulbs, but Im wondering if and how it damages other kinds of bulbs. I know that light bulbs run off of ac current so they are turning on and off 120 times a second so I find it hard to believe that this would damage them.
Incandescent filament lamps resistance changes dramatically - up to ten times - as they warm up. The result is that there is a large inrush current initially but as the element heats the resistance increases and the current decreases to its nominal value. This explains why bulbs generally popped on switch-on rather than at a random moment when burning steadily. (The thermal inertia of the element is high enough that it doesn't cool down between AC mains half-cycles.)
During rapid switching the filament would not cool completely before the next switch-on so it would not be stressed so much. The longer the 'off' periods the harder it is for the lamp.
My domestic experience was that lamps on dimmer switches lasted 'forever' relative to the other lamps in the house. This was due to the rotary dimmer being turned up over, say, half a second and gradually increasing the current while the element warmed up. There was no sudden inrush current.
Lamptech have an article on the subject with some graphs and response times. They also contradict me regarding longer off times. (Ah well.)
That statement originated (to my knowledge) in the damage that can be caused to incandescent lights (those that produce light as a result of [super] heating some element in the light).
Fluorescent lights can also be damaged by repeated on/off switching of the "switch," still due to a startup-effect, but differently than incandescents.