Mirror
Mirrors were polished surfaces which cast reflections. Some mirrors exhibited unusual properties.
In 1866, Edward Waterfield and Theodore Maxtible experimented with static electricity and mirrors in an attempt to develop time travel, and their prototype ended up summoning Daleks from Skaro. (TV: The Evil of the Daleks) Another time machine that used mirrors was developed by General Mariah Learman in the early 21st century. (AUDIO: The Time of the Daleks) Rhian Salmond had a great aunt who time-travelled through powered mirrors. (PROSE: The Book of the Still)
In a parallel world, UNIT used mirrors and the Doctor's TARDIS to create a Lodestone and send Donna Noble back in time. (TV: Turn Left)
Tharils and Mirrorlings could use mirrors to travel to different dimensions. (TV: Warriors' Gate, COMIC: Mirror Image) Similarly, the Silversmith's mirrors were connected to Spherion crystals inside the pocket of unreality that housed Mirror Edinburgh, allowing the Mirror Silversmith and the rest of the Mirror Folk to use them as portals to and from said bubble dimension. (COMIC: The Silvering)
Mirrors were harmful to some life forms; they were used to defeat the Gorgon on Zeno, (COMIC: The Gaze of the Gorgon) another Gorgon on Earth, (TV: Eye of the Gorgon) Medusa in the Land of Fiction, (TV: The Mind Robber) the Mara on Deva Loka, (TV: Kinda) and Mar'tuth in a branch of Homestyle in England. (PROSE: She Came From Another World!) The Borad banned mirrors during his reign over Karfel. (TV: Timelash)
The Tenth Doctor punished Daughter of Mine by trapping her in every mirror. She was said to be the strange movement seen out of the corner of the eye when looking in a reflective surface. (TV: The Family of Blood)
The legendary Mortal Mirror of Castle Extremis was a portal to a parallel universe. Those who passed through it and were observed by someone on their side became trapped in the other realm, only able to return to their own reality as fragile beings made entirely out of glass. (PROSE: Martha in the Mirror)
The Fifth Doctor was able to trap the Mara in a Circle of Mirrors, (TV: Kinda) but the Mara was able to re-possess Tegan and communicated with her via a mirror, being able to as there was no circle. (TV: Snakedance)
The Saturnyns' use of perception filters meant that mirrors would not show their reflections, mimicking the legends of vampires (TV: The Vampires of Venice) while the Krafayis could only be seen in mirrors. (TV: Vincent and the Doctor)
Susan discovered a full-length mirror in the TARDIS wardrobe which reflected a pale-skinned young man with fangs. (PROSE: The Exiles) Later, Ace discovered this same mirror and the same reflection of the young man. (PROSE: Echo)
The Doctor often used a mirror after regeneration to check his new appearance. The Second Doctor looked into a mirror to see what he looked like after his regeneration. At first he saw his new face with dark hair and green eyes, but the image wavered and momentarily became that of the First Doctor's. (TV: The Power of the Daleks) The Third Doctor was initially displeased with his new face when he looked in a mirror, however he quickly grew to like it. (TV: Spearhead from Space) When he looked into a mirror for the first time, the Fourth Doctor found his nose to be an improvement but wasn't sure on his ears. (TV: Robot) The Fifth Doctor was not pleased with his new face, remarking that with regeneration you never knew what you were going to get. (TV: Castrovalva) After his regeneration, the Sixth Doctor was delighted with what he saw in the mirror, declaring his new incarnation was his finest yet. (TV: The Twin Dilemma) Waking up in hospital in a state of post regenerative amnesia after his seventh regeneration, the Eighth Doctor reacted with horror seeing his new face in a room of mirrors, crying out "who am I?" (TV: Doctor Who) Shortly after regenerating from the War Doctor, and ashamed of his predecessor's actions, the Ninth Doctor smashed every mirror in his TARDIS. He vowed never to look at his face, as he contemplated his apparent destruction of Gallifrey and how many children died as a result, as well as how many children he had to save to make up for it. (PROSE: The Day of the Doctor) However, he later checked his reflection in a mirror at Rose Tyler's residence , seemingly pleased with his face, stating it could have been worse but was shocked at the size of his ears. (TV: Rose) The Eleventh Doctor was delighted at the strangeness of his face, calling it a "Moon face". He also identified it as his eleventh face. (PROSE: The Day of the Doctor) The Twelfth Doctor was confused about his face after his regeneration. He initially thought the mirror was furious. When examining his features again, he was further confused, as he had seen the face before, and that there were wrinkles, despite the fact that his face was new. He didn't really like his face, especially his eyebrows, which he thought could be attack eyebrows. (TV: Deep Breath)
The Lady Cassandra had a convex mirror in her basement lair underneath the New New York Hospital, which she used to examine her new body after psychografting her consciousness into Rose Tyler. She was initially horrified, then became rather enamoured of her new appearance. (TV: New Earth)
Mirrors were also heavily involved in superstition. Humans, at least, believed that one would get seven years bad luck for breaking a mirror. The Tenth Doctor remarked on this upon realising that he had trapped himself in the past by breaking a mirror serving as a time window, except in his case it was 3000 years since he had come from the 51st century. (TV: The Girl in the Fireplace)
"Looking-glass" was another word for mirror; when the Eleventh Doctor was about to pass through the secret portal to the Shakri ship with Amy Pond, he remarked, "Through the looking.glass, Amelia?". (TV: The Power of Three)
During the Siege of Trenzalore, the Eleventh Doctor used a mirror to freeze a Weeping Angel by forcing it to look at its own reflection, writing "with love from the Doctor!" on it. (TV: The Time of the Doctor)
The only way Bill Potts was able to see her true cyber-converted self was by using a mirror. (TV: The Doctor Falls)
A mirror in Norway acted as a portal between N-Space and the Solitract plane, with an anti-zone forming in between. (TV: It Takes You Away)