Maths Challenge

100 Lightbulbs

Imagine 100 lightbulbs. 

All have an on/off switch.  All are on.

Switch off every second light.

Flick the switch of every third light, so that if it was on it is now

off, if off it's now on.

Flick the switch of every multiple of four.

Flick the switch of every multiple of five.

Etc. etc. etc...

Flick the switch of every multiple of 99

Flick the switch of every multiple of 100.

Which bulbs are on and which are off?

Extension:
what would be the area of a similar arrow, drawn in a 10x10 square?
Can you generalise for a n x n square?
What about arrows in general?