Math + Making

A student blog for Math 189AH: Making Mathematics at Harvey Mudd College

Making a Pepper’s Ghost

Kishore Rajesh

For my third project, I decided to make a pepper’s ghost illusion, which is basically a kind of “hologram” used in various special effects, most notably in Disneyland’s Haunted Mansion:

What is a Pepper’s Ghost?

A pepper’s ghost isn’t a real “hologram,” per se. It’s basically a reflection of light through a transparent material. Since the material is transparent, we only see the reflection, giving a “ghostly” illusion. A few years ago, I wanted to make a holographic watch, and after doing some research, I decided that a pepper’s ghost was the best way to do something similar.

My Initial Plans

My plan was to take a clear plastic dome (see below), and cut out a cross-section of plexiglass (see below), which is the transparent material the illusion is reflected off of. Then, I can put the dome on top of any video and the video would be projected off and into the air.

Cutting plexiglass is relatively hard, so I decided to use the laser cutter, which is faster and more accurate than handcutting. However, this means I’d need an SVG file to use with the laser cutter for the plexiglass. I decided to approximate the dome as a cylinder, so a cross section angled up would just be an ellipse! After getting some measurements on the dome, I found the major and minor axes of the ellipse, as well as a way to cut off a side of the ellipse so that it can stand on the bottom of the dome. My somewhat-incoherent work on this is below:

Using the Laser Cutter

This was my first time using the laser cutter by myself. I had used it before in project 2 when making a demo for a tensegrity model, but my partner had manned the computer and machine. However, after reading the manual for the GlowForge, I figured out how to use it. I just had to put the material I wanted to cut in the machine, close the lid, and drag and drop my svg file into the computer program. Half a minute later, I’d have a perfectly cut ellipse.

Making the Final Product

To test my laser cutting skills, I cut the ellipse on a piece of scrap wood from the makerspace. To my chagrin, it was too large for the dome. I tried a few other times, slightly increasing and decreasing the dimensions of the ellipse, until I eventually got a good enough cut.

I then moved onto cutting the plexiglass. The first try was somehow too small (I attribute this to the wood being thicker than the glass, making it fit). The second cut was a lot better, and fit snugly into the dome. All I had to do was remove the paper coating from the glass and I’d be set! I went back to my room to do this part since I was done with the laser cutter.

That was easier said than done. First of all, the paper was pretty stuck on there, so I had to put in some effort to peel it off, which took longer than I’d like to admit. Then, when I tried to put the glass back in the dome, it didn’t go in fully. I tried gently bending it a bit and the glass snapped in half. Back to the drawing board (makerspace).

Luckily, after I went back to the makerspace, the first piece I cut fit into the dome. I then peeled off the paper coating *in the makerspace* and it still fit. So, I was done making the Pepper’s Ghost contraption!

Overall, I had a lot of fun making the pepper’s ghost, both coming up with the idea and using the laser cutter! In the future, I’d like to make more pepper’s ghosts, perhaps ones small enough that they could be added to a watch?

Here’s a video of the pepper’s ghost in action with a cool Isocohedron animation:


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