This project shows how to build a laser projector using an Arduino, a heptagon pillbox, a cooler fan and a laser pointer. Check how it works and construction details on the link below. The heptagon pillbox scan the projection surface and produced the words. [via]
Because the mirrors are moving the reflected laser dot sweeps the screen from left to right, because it does it very quickly your eyes actually see an horizontal solid line, in this case you see 7 lines one on top of the other because the each mirror is at an angle.
DIY laser projector – [Link]