|Muggles' Guide to Harry Potter - Magic|
|Features||Changes the form of a boggart|
|First Appearance||Harry Potter and the Prisoner of Azkaban|
Riddikulus is a spell used against boggarts. The caster must think of something funny while casting this spell. If successful, it forces the boggart to take the shape of whatever the caster was thinking about. As boggarts feed on fear and are defeated by laughter, the object of the spell, ultimately is to make the boggart laughable.
We see this spell invoked several times in Harry Potter and the Prisoner of Azkaban when Professor Lupin is teaching the spell to his Defence against the Dark Arts class. While Professor Lupin does provide a Boggart for use in Harry's anti-Dementor lessons, we don't see the Riddikulus spell being used against it. The spell appears again in that book when a Boggart is one of the tests in the final exam.
A Boggart also appears in Harry Potter and the Order of the Phoenix. Mrs. Weasley is trying to use the Riddikulus spell against it, and is repeatedly failing. The change of aspect forced by the spell, however, allows Harry to note that Mrs. Weasley cares as much about his own welfare as she does about any of her family.
This spell is of quite limited usefulness, being effective against Boggarts only, at least within our story arc. That said, it is used, as mentioned, a number of times in the series. The fact that Professor Lupin is able to find Boggarts on demand, at least twice and possibly three times during the school year, does indicate that they are relatively common in the Wizarding world. (The story does not specify whether the Boggart used in Lupin's practical final exam is the same one Harry was using for his anti-Dementor spells.)
The author is mute on how Boggarts can be handled in the case where there is no real way to make one's greatest fear seem funny. We see two cases of that in the series: Lupin's greatest fear being the full moon, and Mrs. Weasley's greatest fear being the death of her family members. We note that Lupin does not try to use the Riddikulus spell himself, instead using some wordless form of magic to force the Boggart back into its hiding place, on the two occasions where we see him acting against one.
As an aside, we note that Lupin used up his one and only (at that point) Boggart teaching the Riddikulus spell to the Gryffindors. We wonder what lesson he taught the Slytherins, Ravenclaws, and Hufflepuffs.
- What would be the effect of casting Riddikulus on a non-Boggart?
One main reason for the Boggart's existence, as mentioned on the write-up of that creature, is to provide an incontrovertible method of displaying the characters' deepest fears. As such, there must be some spell to fight them. Given that the Boggart inspires fear so as to feed on it, that which destroys it must be the antithesis to fear. Having the Boggart be defeated by laughter is, in fact, quite logical, but the author's choice of the spell and its results, and her choice in handing it to Neville for the first and the most effective invocations, seems somehow inspired.