Well, IRL there would be no visible beam, more like point, click, and target bursts into flames. A visible laser beam probably would work off of a different principle. Probably would be some sound though, ignoring the screams of your target, there would be crack of thermal noise generated by the area quickly being heated up. And since Fallout is like 225+ years in the future, we wont need a Boeing 747 to lug around a 1-shot Lascannon lol