In Harry Potter, Do Spells Travel in a Straight Line?

Question

In the Harry Potter world, when a magical person casts a spell, sometimes they target a specific object, like when Harry has a wand only and needs his broom, he casts "Accio Firebolt" and his broom comes to him.

But a lot of the time people point their wands just in the direction of someone to cast a spell. One example would be in a duel. When they do this, are they casting a spell on a particular person or at that person? It seems, from what I remember of the movies, that some spells travel in a straight line while others specifically target a person or object.

Do any spells travel in one direction, like a laser beam? What kind of spells are directional (instead of focusing on a particular person or object)? And can directional spells be deflected or blocked by something like a mirror or another physical object?

(By "directional," I could have said linear, but didn't want to start sounding mathematical or scientific. In other words, do they travel in one direction, like a laser beam would, or will they change direction to hit the intended target?)

Answer 1

It is my interpretation of canon that not all spells are inflexible per se, meaning some can bend and move

Not all spells travel in just one direction. For example: Protego Totalis, Repellum Muggletum, Muffliato, and Priori Incantatem create what I can best describe as a dome-like effect, encasing persons or an area. The first two spells are protection spells that Hermione uses in Deathly Hallows, Muffliato is a spell that keeps those around you from hearing a conversation you're having with others (Harry, Ron, and Hermione used it in Half-Blood Prince and Deathly Hallows). We're probably all familiar with Priori Incantatem and how it created a cage of light over Voldemort and Harry in Goblet of Fire. Hominum Revelo is a spell that Hermione casts just inside the front door of No. 12 Grimmauld Place, which reveals there are no other persons within the entire house. In Prisoner of Azkaban, Harry produces a Patronus, which moves freely about once its in corporeal form. The Patronus doesn't require that a person aim at another or a being. Harry Accioing his broom, as stated in your question, is a great example.

A literal example of a directional spell is Point Me, which tells the caster where he or she is.

There are many examples of spells aimed at a specific person or being being deflected or reversed. In Chamber of Secrets Ron's defective wand shoots the slug-vomiting curse back at him, rather than straight forward to Malfoy. In Goblet of Fire, Harry's spell connects with Voldemort's and keeps it at bay, forcing Voldemort's spell into a kind of stasis. When Harry jerked his wand up, breaking Priori Incantatem, Voldemort's Avada Kedavra spell did not continue on toward Harry, so one could say Harry's Expelliarmus blocked Voldemort's spell. In Order of the Phoenix, Dumbledore's duel with Voldemort featured a lot of blocked spells and spells that did not hit their intended target.

But the headless golden statue of the wizard in the fountain had sprung alive, leaping from its plinth to land with a crash on the floor between Harry and Voldemort. The spell merely glanced off its chest as the statue flung out its arms to protect Harry.
_{Harry Potter and the Order of the Pheonix, chapter 36}

It's also in Order of the Phoenix that Fawkes actually swallows one of Voldemort's Avada Kedavra curses, dies, and is reborn. Prior to Voldemort and Dumbledore's duel, the DA members battle the Death Eaters in the Department of Mysteries and curses are deflected, misdirected, and incomplete, and they certainly do a lot of damage to the Hall of Prophecy and the Time Room, breaking and destroying myriad items, and the spells stop once they hit a target (even if it's not the intended target). In Deathly Hallows, Hermione's Confringo spell cast at Nagini rebounds from inside Bathilda Bagshot's house and hits Harry's wand, breaking it.

Protego is a blocking spell that a person can cast against an incoming spell. I don't think it would work against Avada Kedavra, because in Goblet of Fire, Barty Crouch Jr. as Mad-Eye Moody states that there is no known countercurse to Avada Kedavra and that it cannot be blocked. It can, however, miss its target.

So, some spells do not travel only in one direction. Spells that do travel in one direction can miss their target and hit something else, which completes the spell. Spells can rebound sometimes. And they can be deflected in a number of ways.

Answer 2

It appears from the books that there are directed spells and undirected spells, sort of like transitive and intransitive verbs. Directed spells are primarily those that come out of the end of the wand and act on a particular object, like most dueling spells - how often does someone dodge a spell, or just miss? Transfiguration spells seem to require pointing at the object too. Accio I'm not sure about - it obviously doesn't need to be exact, perhaps either the range expands like a flashlight as you get further away, or maybe it doesn't matter where you point your wand at all, you just need to focus on the object. Then there are other spells that just affect an area - protective charms, for instance. They don't really have a target, they just affect the general environment.

Directional spells can definitely be blocked, we see spells hitting walls or tombstones, for instance, and even Voldemort's avada kedavra was blocked by the statue Dumbledore enchanted. I'm not sure about mirrors, I believe some spells would be reflected, but others (particularly strong or evil spells) would just break the mirror.