The Shining (1980) is full of mysteries and ambiguities, but there is one constant: If Jack (Jack Nicholson) is seeing a ghost, there’s a mirror nearby.
A full-length mirror can be found in room 237. Behind the bartender, there’s a bar mirror. During Jack's chat with Delbert Grady, there are mirrors in the bathroom, and the two are standing on a shiny, polished floor capable of reflection. Is there a reason for all this?
Many people speculate that the ghosts aren’t real, and the mirrors represent the fact Jack is talking to himself when he sees these visions of menace. He’s the only true danger within The Overlook, to himself and his family, so he’s projecting his evil. But the theory doesn’t quite hold up - Danny (Danny Lloyd) and Wendy (Shelley Duvall) both see apparitions from the hotel’s past without the use of mirrors, and they are good people.
It’s more likely the use of mirrors (conjoined with the film's acute attention to overall aesthetic symmetry) are representative of the nature of duality in the film’s characters and themes. A tremendous number of shots focus on visual symmetry (two girls standing at the end of a long hall, the corridors through which Danny rides his bike, the hotel lobby where Jack’s writing desk sits), and mirrors are used in various ways beyond Jack’s communication with ghosts (take Danny’s mirror-based conversations with his finger friend Tony, for instance).
The Shining is all about doubles. Jack himself is evidenced to be a reincarnation of a previous person, and the film is replete with cycles. Danny has Tony as his representation of duality. Tony takes him so far as to write “murder” on the door in the form of “redrum,” a mirror image.
“The assorted mirror concepts in The Shining aren’t just about disorientating us. They are essential to unravelling the hidden narratives of the film, through the concept of duality. This duality takes many forms – character duality, location duality, scene repetition, and of course, parallels between the film’s content and the audience’s reality – a standard Kubrick device.” - Collative Learning