"It shows us nothing more or less than the deepest, most desperate desire of our hearts. You, who have never known your family, see them standing around you. Ronald Weasley, who has always been overshadowed by his brothers, sees himself standing alone, the best of all of them. However, this mirror will give us neither knowledge or truth. Men have wasted away before it, entranced by what they have seen, or been driven mad, not knowing if what it shows is real or even possible."
-- Albus Dumbledore (PS12)
The Mirror of Erised is a magnificent mirror, as high as a classroom ceiling, with an ornate gold frame, standing on two clawed feet. The inscription carved around the top read “Erised stra ehru oyt ube cafru oyt on wohsi,” which is “I show not your face but your heart’s desire” written backwards (that is, in what is called ‘mirror writing’).
The mirror itself was very old, and it’s origins unknown. It was created in a spirit of fun, as it was more revealing than a normal mirror, but not really useful. A teachers might have brought it with him from his travels. The mirror had been languishing in the Room of Requirement for a century before Dumbledore changed it into a perfect “hiding place and final test for the impure of heart” (Pm).
In 1927 the Mirror of Erised was stored in the Room of Requirement under a black velvet curtain. Albus Dumbledore went to see it after not looking into it for many years. He sees himself and Grindelwald as teenagers facing each other in a barn (CG73).
When someone looks into the mirror, they see the deepest, most desperate desire of their heart. The mirror has trapped people who can't bear to stop staring into it, unsure if what they see is going to actually happen. Harry was therefore warned by Dumbledore to stop looking at it. But even when Harry stopped, he kept having nightmares about it (PS13).
In the mirror, Harry saw his family: his father and mother, but also at least eight others, including one of his grandfathers. Ron saw himself as Head Boy and Quidditch champion (PS12).
Dumbledore used the power of the Mirror to protect the Philosopher's Stone:
"It was one of my more brilliant ideas, and between you and me, that's saying something. You see, only one who wanted to find the Stone--find it, but not use it--would be able to get it, otherwise they'd just see themselves making gold or drinking Elixir of Life. My brain surprises even me sometimes..." (PS17)
Harry wanted only to find the Stone when he looked into the Mirror. As he did not want to use it, he suddenly found it in his pocket (PS17).
Harry thought of the mirror again when revisiting his memories with Snape during Occlumency lessons (OP24) and when looking for Sirius after his death (OP38). He hoped the two-way mirror would show him Sirius, just like the Mirror of Erised showed him his parents. Unfortunately, that did not happen.
Dumbledore also mentioned the Mirror of Erised again, when pointing out to Harry how rare it was for someone not to see themselves being immortal or rich (HBP23).
Dumbledore returned the mirror to the Room of Requirement, and it, along with everything else being hidden there, was destroyed by Fiendfyre in the Battle of Hogwarts (Pm).
Dumbledore said that he would see himself holding a pair of socks, when looking in the mirror (PS12). Harry suspected that Dumbledore was noT telling the truth at the time (PS12, DH2). Only upon meeting Dumbledore at King's Cross (DH35) did Harry find out what Dumbledore really would have seen in the mirror. However, even there it was not described. Dumbledore would see his family whole, happy and reconciled, Rowling revealed only after the final book had been published (BLC).
If Voldemort had ever come across the Mirror of Erised, Rowling stated he would have seen himself all-powerful and eternal (TLC). Before Harry went on the search for the Horcruxes, he would have seen Voldemort defeated, as he would have had no peace before that had happened (TLC).
The inscription of the mirror was never explained in the books; Rowling explained it on Pottermore. She also added that Dumbledore's warning mirrored her own views. Holding on to your dreams may become unhelpful and unhealthy, when you cling onto dreams that do not (or should not) come true.
"Erised" is "desire" spelled backwards. The word mirror comes from the Latin 'mirare', which means 'look at
In a Scholastic interview, Rowling mentioned that the chapter on the Mirror of Erised was one of her favourite chapters. She also told that she had enjoyed reading the essays about what students would see in the Mirror (Sch2). Rowling herself would probably see her mother (BN). She would have loved to have more time with her mother, but also knows it would never have been enough (HPM). She also jokingly hoped to see a scientist inventing a healthy cigarette and a journalist being boiled in oil (RAH).
Rowling also stated:
"I was conscious that when I looked in the mirror, I would see exactly what Harry saw. But it was only when I'd written it that I fully realised where it had all come from. It is an enormous regret to me that my mother never knew about any of this, second only to the fact that she never met my daughter." (ET)
Rowling's statement that the Mirror was kept in the Room of Requirement is problematic, as according to Harry, "Dumbledore and Flitwick, those model pupils, had never set foot in that particular place" (DH31). So how did Dumbledore get the mirror in the first place? Rowling also never mentioned what happened to it after the end of Harry's first year. He does not come across the Mirror in the Room of Requirement in his sixth or seventh year.
In the first film, only Harry's parents are shown in the Mirror, instead of his whole family. The filmmakers had actually asked Rowling herself to play Lily Potter in this Mirror of Erised scene (jkr.com rumours), but Rowling refused, stating she would have messed it up somehow. The Mirror is shown at both Hogwarts castle in the Wizarding World of Harry Potter in Orlando, Florida, and in the Studio Tour in London.
Mirrors and the magic of reflections are used frequently in fairytales and myths. Most mirrors showed the truth: in Snow White, it told the truth about who the most beautiful girl in the world was. In Beauty and the Beast, it showed the condition of Belle's father. And in Oscar Wilde's Picture of Dorian Gray, it even showed the soul of the person in front of it (Wikipedia). Wasting away in front of mirrors is also well-known in literature. For example, in Greek mythology, Narcissus "wasted away" while gazing at his own reflection in water (Wikipedia).
In different languages, translators have struggled with the Desire/Erised translation. A small sample of the translations is shown below:
Mirror of Erised in different languages
Czech – Zrcadlo z Erisedu
Danish – Drommespejlet
Dutch – Spiegel van Neregeb
Finish – Iseeviot-Peili
French – Le miroir du Riséd
Friesian – Spegel fan Negnalref
German – Der Spiegel Nerhegeb
Hungarian – Edevis tükre
Italian – Lo Specchio delle Brame
Latin – Speculum Erisedii
Polish – Zwierciadlo Ain Eingarp
Portuguese – O Espelho dos Invisiveis
Spanish – El espejo de Oesed
Vietnamese – Tam guong Ao anh
Welsh – Drych Uchwa
The information of the mirror's destruction comes from the new Pottermore ebook.