Lasers appear in Chapter 2 - The Hospital in levels 2-16, 2-17, 2-18, 2-19, and their dark world counterparts as well as other chapters (but in different variations). Lasers kill Meat Boy upon contact, and they turn on and off.
*In Chapter 4 - Hell, there are certain lasers that shoot out of what looks like a robotic eye, and slowly point towards Meat Boy .