During our visit to Walt Disney World, the new wife and I made sure to hit all the classic rides in the Magic Kingdom: Pirates of the Carribean, The Tiki Room, The Haunted Mansion, even It’s a Small World (though, alas, not Space Mountain, which is under renovations until November).  The Haunted Mansion is one of my favorites, with its classic Gothic ghost story atmosphere and dark sense of humor.  As a child, I was terrified of the essentially harmless attraction.  This trip, as a professor of optics, I was delighted to not only see the clever special effects, but deconstruct them — to “peek behind the curtains”, so to speak.

I suppose some would think that this peek would “ruin the magic” or “unweave the rainbow“.  For me, though, I find it a joy to see how people’s ingenuity can lead to wonderfully fun, even beautiful, attractions.  The Haunted Mansion is filled with clever applications of very simple optics, and I can’t resist explaining one of them.

SPOILER ALERT!  IF YOU FEEL THAT UNDERSTANDING HOW AN ATTRACTION WORKS RUINS IT, DON’T READ ANY FURTHER…

The attraction mostly consists of the visitors riding through the shadowy halls of the mansion in seats known as “doom buggies”.  Early on, the carts pass by and face into a great hall, within which the visitors can see ghostly figures enjoying a great party, both sitting at the table and circling the chandelier (pictures from the blog Disney and More):

The figures appear fully three-dimensional, transparent, and in motion.  A detail, also from Disney and More, is below:

So how is it done?  A natural, and common, reaction is to assume that somehow it is done with some sort of projector, or hologram.  A projected image would not appear three-dimensional to all visitors passing the dining hall, however, as the scene can be viewed continuously as the “doom buggy” passes the hall.  Holography is a common guess, but the mansion opened in 1969, and the first holograms had been made only a few years earlier, in 1962 — dynamic, multicolored holograms would be years away.

The real answer is much more mundane, and actually much more elegant because of it: it is simply a reflection!

If you look at the first picture above, you can see a pillar partially obstructing the view.  The pillars in the room are actually supports for panes of glass which are situated between the visitors and the dining hall.  The “ghosts” which appear are actually animatronic figures which lie in hidden nooks directly above and below the doombuggy track.   When the figures are illuminated, their reflections can be seen by the visitors, and appear to be sitting in their seats, circling the chandeliers, etc.:

The simplicity of the setup is what makes it so wonderful.  The ghosts look three dimensional because they are reflections of three-dimensional objects!  Placement of the ghosts is also simple: if you want a ghost to appear 10 feet beyond the pane of glass, simply place the ghost figure in its “mirror image” location 10 feet in front of the pane of glass.   The Disney and More site mentioned earlier confirmed my suspicions about the workings of the ballroom, and included this picture of the “behind the scenes”:

I illustrated the idea in practice to my new bride while we were out at dinner.  We were sitting by a big window, and I pointed out that the reflection off of the glass made it look as if a “ghostly bride” were sitting right outside the window.  If I placed a real chair outside in that exact position, I would have a very rough imitation of the mansion’s effect.

Once you understand this effect, you understand why Disney prohibits flash photography in the mansion, and why it is a waste of time to take such pictures.  The light from a flash would reflect off of the glass pane, and a flash photo of the dining hall would only produce a very lovely image of the photographer’s flash.

I should note that, as with all good special effects, the devil is in the details: the lighting in the dining hall has to be done just right to achieve that ghostly look, the animatronics has to be hidden so it doesn’t have its own ghostly image, objects which obstruct the view of the ghosts must have their own, black-painted mirror image counterparts, etc.  For me, though, it is a joy to see how some very simple optics can be used to make a very wonderful effect.

It is also worth noting that this illusion is an awesome implementation of an effect known as Pepper’s ghost, which was developed originally back in the mid-1800s.