Normally invisible. Spells bouncing off of the shield give off their own light.
Magically blocks spells & physical entities
The Shield Charm (Protego) is a term applied to several varieties of charms. They create a magical barrier to deflect physical entities and spells, in order to protect a certain person or area. Conjurations may sometimes rebound directly off it back towards the caster or in other cases, may ricochet off in other directions or dissipate as soon as they hit the shield.
It's unknown who the inventor of the spell was and when it was invented. Harry Potter learned this spell in his fourth year, in preparation for the third task of the Triwizard Tournament. Harry also taught this charm to Dumbledore's Army during the 1995-1996 school, so the members would be able to protect themselves when the time came. This was also the subject of a question of the Theory of Charms O.W.L. in 1996.
This spell was used frequently during the Second Wizarding War, the Battle of the Department of Mysteries, the Battle of the Astronomy Tower, the Skirmish at Malfoy Manor, the Battle of Hogwarts, as well as on many other occasions.
Casting and effects
When cast, a (usually) invisible shield appears where the casters wand is pointed providing a protective barrier between themselves and their attacker. The shield itself does not give off light, but rather, the spell bouncing off of it does. The only spell the Shield Charm can't defend against is the Killing Curse, since it's unblockable.
The Shield Charm is a moderately difficult spell. Fred and George Weasley realised that most adult wizards, even those employed by the Ministry of Magic, could not produce a functional Shield Charm (something that not only could Harry Potter accomplish at age fourteen but, also impart to members of the D.A.). They used this to their advantage when marketing their line of protective clothing (hats, cloaks, etc.), upon which permanent Shield Charms were cast.
The incantation for the simplest Shield Charm spell is Protego.
other varieties of the Shield Charm include as followed:
Harry Potter first learned this spell in his preparations for the Third Task of the Triwizard Tournament, during which his friends Ron Weasley and Hermione Granger shot spells at him so that he could practice shielding himself. Although he later demonstrated an impressive mastery of this technique surpassing that of many Ministry employees, Harry initially had difficulty casting this spell as "Hermione was able to shatter it with a well-placed Jelly-Legs jinx."
Harry once used the charm to defend himself against Professor Snape's Legilimency during Occlumency training, and found himself unexpectedly seeing some of Snape's memories. This was most likely the reason why Snape was using a Pensieve to store his memories, and thus to preserve his privacy. 
|Harry used the spell multiple times during the Battle of the Department of Mysteries in order to protect himself and his fellow D.A. members from Death Eaters, and to prevent them from summoning the Prophecy from him that night.|
|During his first sixth year Defence lesson, Harry and the other members of his class had to cast this spell non-verbally. When Snape tried to jinx him, Harry instinctively used a Shield Charm (verbally cast) that was powerful enough to throw Snape off his feet, against a table.|
Harry cast this charm between Hermione and Ron after the latter returned, since Hermione was furious with Ron to the point of wanting to curse him.
|Harry used the charm multiple times during the Battle of Hogwarts to protect various fighters on his side, such as Molly Weasley after she killed Bellatrix Lestrange. Harry's Shield Charms were so powerful they were able to block curses from Lord Voldemort himself.|
|1489||Hannah Cockleford used the Shield Charm to protect a Muggle boy named Edmund Gaddlegate when he was forced to joust with the Earl of Paunchley.|
|1992||Gilderoy Lockhart presumably attempted to demonstrate this charm, or something similar, to Harry during the first meeting of his duelling club, but all he truly wanted was to twirl his wand fancifully, which failed spectacularly, dropping his wand in the process.|
|1995||This charm was among those that Harry taught the members of Dumbledore's Army in his fifth year.|
|1996||Dolores Umbridge used a shield charm to protect herself from the centaurs' arrows in the Forbidden Forest.|
|During the Battle of the Department of Mysteries, the fighting Death Eaters cast many Shield Charms to protect themselves from Dumbledore's Army's Stunning spells.|
|Lord Voldemort used a Shield Charm to defend himself during his duel with Albus Dumbledore.|
|Hermione Granger managed to cast this nonverbally in their first sixth year Defence Against the Dark Arts lesson.|
Severus Snape cast this charm to deflect Harry Potter's spells during their brief duel after he killed Albus Dumbledore.
|Remus Lupin, Nymphadora Tonks, and various other guests at Bill Weasley and Fleur Delacour's wedding cast shield charms after Kingsley Shacklebolt's Patronus arrived to warn those at the Burrow that Lord Voldemort had taken over the Ministry of Magic.|
|Hermione Granger cast the Shield Charm when Harry and Ron got into an argument during their search for the Horcruxes. Hermione cast the shield before they could draw their wands against one another. When Ron subsequently left, Hermione was hampered in trying to follow him and persuade him not to leave by her own shield.|
Hermione Granger used a Shield Charm during the 1998 Gringotts break-in, to disrupt Thief's Downfall.
|A shield was casted around Hogwarts just before the final battle in the hopes of delaying Lord Voldemort and his army, in order to give Harry Potter time to find the Horcrux hidden there.|
|Voldemort also used many Shield Charms when duelling Harry Potter and many others during the Battle of Hogwarts.|
Latin protego, "I cover" or "I protect".
Behind the scenes
- In the films, Shield Charms are used in nearly every duel shown, almost always used non-verbally and usually accompanied by a parry-like wand movement.
- Also in the films, Shield Charms are shown to be able to deflect material objects such as arrows which were shot using a bow. Also it has shown to be able to withstand dragon fire from a Hungarian Horntail as seen in Harry Potter and the Deathly Hallows:Part 1 The Video Game.
- The book depicts the Shield Charm as only being capable of repelling "minor to moderate curses and hexes", although Harry Potter uses it to block the worst effects of Antonin Dolohov's curse in the Department of Mysteries. However the film implies the Shield Charm to be significantly more powerful in that Harry is able to use it to completely deflect powerful curses cast by Voldemort himself.
- In the early video games, Expelliarmus functions as a shield charm, due to a lack of enemies to Disarm. In these games, Protego is absent.
- In the books Protego is also able to keep two quarrelling wizards from going at each other when a shield is cast between them.
- In Harry Potter and the Deathly Hallows: Part 2, Protego is shown several times to be able to block the Killing Curse. Harry blocks multiple Killing Curses from Voldemort during their final duel and Bellatrix Lestrange blocks four Killing Curses while duelling Molly Weasley. According to the books this is incorrect as the Killing Curse is "unblockable" which is the main reason for its particular infamy, but within the context of the films the fact is never mentioned. As the Killing Curse would instantly end any duel if it were unblockable, this might just be done for dramatic effect and for longer duels.
- In Harry Potter for Kinect Harry and Voldemort can use this spell against each other in the final battle during Voldemort's Last Stand as they engage in a normal duel.
- Harry Potter and the Goblet of Fire (First appearance)
- Harry Potter and the Order of the Phoenix
- Harry Potter and the Order of the Phoenix (film)
- Harry Potter and the Order of the Phoenix (video game)
- Harry Potter and the Half-Blood Prince
- Harry Potter and the Half-Blood Prince (film)
- Harry Potter and the Half-Blood Prince (video game)
- Harry Potter and the Deathly Hallows
- Harry Potter and the Deathly Hallows: Part 1
- Harry Potter and the Deathly Hallows: Part 1 (video game)
- Harry Potter and the Deathly Hallows: Part 2
- Harry Potter and the Deathly Hallows: Part 2 (video game)
- LEGO Harry Potter: Years 5-7
- Harry Potter: Spells
- Pottermore (Mentioned in a wood carving at Spells/Duels)
- Wonderbook: Book of Spells
- Harry Potter for Kinect
Notes and references