I know this is a bit of a silly question, but I have to ask it as a scientist: How far does this spell (and others I guess) penetrate objects?
Clearly it goes through clothes, from the books, but I also remember Harry leaping behind tombstones in the graveyard when Voldemort and co. were trying to kill him, and they apparently didn't go through those. At a further extreme, it probably can't just go through walls and kill people.
So what are its limits? Do you just have to see the person or something? Or is it kind of like a gun?