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Pepper's Ghost Illusion

The "Pepper's Ghost" illusion is an old stage trick to create a semi-transparent "ghost" appear before an audience. This is accomplished by having an illuminated off-stage subject reflect off an angled pane of glass. You know how if you put your face up against a window at night, you can kinda see your own reflection? That's the basic premise.
Despite being created in the Victorian Era, it still holds up today as a dazzling trick! I can only imagine what audiences thought of it back when it first debuted in the 1860s.
Pepper's Ghost is still used today in various shows, circuses, and amusement parks to great effect!
The most famous use of Pepper's Ghost today is in Disney's Haunted Mansion rides. At one point in the ride, semi-transparent ghosts appear to dance in a large ballroom:
These animatronics are actually off-stage (below and above the ride), and are being reflected off a giant glass pane.
Circuses sometimes have a "Girl-to-Gorilla" sideshow. Since the off-stage subject only appears when illuminated, Pepper's Ghost can be used to create a "transformation" type effect by fading the lights. In these sideshows, a woman transforms into a gorilla (see a demonstration in this James Bond scene).

Disney's Winnie the Pooh ride also utilizes this "transformation" effect when Pooh falls asleep:
Instead of "transforming" into a gorilla, Pooh appears to have an out-of-body experience as he floats into a dream. In reality, the lights fade on the on-stage Pooh while Pooh's "ghost" is simultaneously illuminated. If you look closely, you can see the seam on the ground where the glass pane meets the floor.
Pepper's Ghost is also how many "hologram" concerts work, too. When the Gorillaz, Michael Jackson, or Tupac perform, a version of Pepper's Ghost is used:
There's some conflicting information online, but a version of Pepper's Ghost appears to be used for Hatsune Miku, at least sometimes. Other times, she's just projected directly on a glass pane, so it's not really Pepper's Ghost (plus, the projector is visible, boooo!)
Pepper's Ghost is also used in movies! In Home Alone, CGI wasn't good enough to make realistic fire yet, so Pepper's Ghost was used to impose flames on Joe Pesci's head:

Pepper's Ghost is how teleprompters work. Here, you can see the monitor below the camera:
Presenters read the "ghost" words reflected on the angled glass in front of the camera lens.
Pepper's Ghost shows up all over the place! Wherever a "hologram" is needed, it's probably a Pepper's Ghost. Check out this "hologram" in a Ghostbusters Pinball machine:
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Planchette cursor by Hillhouse