You’ve probably seen an old Western movie where the good guys settle down and run a string at ankle height around their camp, tying it to a can filled with rocks. When the bad guys try to sneak up in the middle of the night, they kick the wire and pull the can over, making a rattle that awakens the sleeping good guys, who win the day. A laser security system works along the same principle. Instead of a string, there’s a beam of light surrounding the area, and instead of a can of rocks, there’s an alarm of one sort or another.
