As all savvy Potter fans know, underage magic is a serious offense in the Wizarding World. However, Reddit user Bride_Zilla3 asks a puzzling question about a spell Harry casts in the opening sequence of The Prisoner Of Azkaban.
"How come Harry is allowed to use "lumos" outside of school at the beginning of PoA? Is it because he's hiding it and it's not really "magic" (muggles would think it's a torch)?"
Hm. Good question.
You can watch the sequence she's talking about right here:
Later on, in The Order Of The Phoenix, Harry almost gets expelled from Hogwarts for performing the Patronus charm in front of his cousin Dudley. Harry himself knows this, since he already received a warning for a Hover Charm, which Dobby the House Elf performs at the Dursley's in the beginning of Chamber Of Secrets.
In the book version of Prisoner Of Azkaban, Harry doesn't cast 'Lumos' at all. (He does perform underage wandless magic later in both the book and movie when he blows up Aunt Marge. He doesn't get in trouble for that one, either, probably because it was accidental wandless magic, which the Ministry doesn't consider such a big deal.) So how does he get away with 'Lumos Maxima' in the movie?
The Spell Didn't Really Work
Here's my theory: for starters, he's not just casting regular old 'Lumos,' a spell we know he knows how to do. He's trying 'Lumos Maxima,' a stronger, more advanced version of the same spell. And in the sequence, he doesn't actually make the spell work.
Maybe the ministry can only detect successful spells and charms. That would make sense, since they probably don't have the time or resources to go tracking down every single unauthorized person who tries a little magic.
He Was Just Doing His Homework
Another Reddit user, SoYoureALiar, offers this explanation instead:
"I'd like to think that perhaps incoming third years were given an assignment by Flitwick to practice that particular spell or something, and thus the Ministry would be notified beforehand."
The Ministry Just Didn't Care
In the beginning of Prisoner Of Azkaban, Sirius Black has just escaped. The Ministry has their hands full trying to track him down. Catching young wizards practicing simple spells is probably low priority.