Holy Holograms, Batman!

Here in St. Louis, we have what’s called the Science Center, which is basically a cool, interactive science museum. There’s this one exhibit that I’m trying to figure out how to reproduce.
You look into this box and see a golden spring. It looks like a real spring, 3d and everything. But when you reach your hand in to touch it (like you’re supposed to), you find that there’s no spring at all; your hand passes right through it.
However, if you go down and back towards you, you can feel the actual spring. (More clearly, you stick your arm in to touch the spring, palm down, then curl your hand down and reach your hand under this shelf thing.)
Initially, I thought it was a hologram. But having the actual spring there would be unnecessary for a hologram. Also, I’ve been researching holograms, because I’m planning to make one for our science fair, and would like to recreate this illusion (using a different object). The problem is, the two major hologram techniques I’ve found are transmission and reflection, and neither seem to be just like what’s at the Science Center. A transmission is closest, but most sites say your eye has to be right behind the holographic plate. Therefore, the Science Center’s exhibit couldn’t be a transmission hologram, because you can reach right out and ‘touch’ the spring, there’s no plate in the way. It couldn’t be a reflection hologram, either. So, is it some special kind of transmission hologram, one that doesn’t have to be looked at through the holographic plate? Or is it not a hologram at all, since the actual spring is included in the system? I thought it may be some sort of lens+mirror trick producing a real image, but the spring is not an actual source of light, and the whole display is actually dark.
Does anyone know how to do that, or could offer some tips on my project?

You have encountered the wonders of 1:1 optical imaging. It’s not a hologram. If you place an object two focal lengths from a concave mirror you will have an image that is also two focal lengths from the mirror, only inverted. Both images will be the same size.

So if you build a box two focal lengths from your concave mirror and put your object (well lit)upside down in that box you will have an image that seems to be on top of the box. It will appear to be 3D because the imaging works independently for each eye. (You have one open side on the box, which faces the mirror, of course. Unless you know what’s going n you think that box is just a pedestal for your spring, or whatever the object is.) Two focal lengths add up to the radius of curvature of the mirror, so what you’re doing is placing your object so its base sits at the center of the sphere.

There are other arrangements of mirrors that will produce the sam result – Edmund Scientific sells a visual illusion that consists of tw mirrors tha fit atop each other, making something that looks like a flying saucer. An object placed inside is perfectly imaged above, and your hand seems to “pass through” that image. You can do the same thing by placing an object at one focus of an ellipsoidal mirror. There wil be a image at the other focus.

I’ve seen an example like the one you describe, nly the one I saw had a vase of flowers as the object. There was an upside-down vase of flowers and a light bulb inside the box that seemed to be the “base”
The limitation in these cases is the quality of the mirror used. f the mirror of good quality the illusion is startling. If not (those Edmund jobs are just vacuum-coated plastic. They can have a lot of distortion)it’s not so impressive.

Argh, CalMeacham beat me to the punch while I looked up the Edmund Scientific gizmo! :slight_smile:

Anyway, here’s the Mind Boggling Optic Mirage from Edmund Scientific …

Arjuna34

There was an arcade video game in 1991 called Hologram Time Traveler that used this same principle to create the illusion of 3D images from a 2D monitor image. Despite the name, no holograms were used.

Here is a link from Science Hobbyist that can help you make a home-made hologram.

I’ve seen the Time Traveller video game. It uses the same optrics as the device I described above, but it’s not really 3D, since it is imaging a flat TV screen. There is an impression of 3D, however, since that flat image seems to be floating in space. An interesting game (a Texas-drawling gun fighter is transported through different milieus. All characters were generated using dressed-up actors, not computer-generated. I was never interested enough to pump quarters into it more than once or twice, though.

Am I the only one for whom a pilgrimage to Edmund Scientific was the high point of the school year??? God, that place was incredible- at least it was 25 years ago. I pray it’s only become better.

We have one in the MSU physics department (on display right next to the real holograms… Some of those things are so real it’s scary). Ours uses a light bulb, and one of the favorite tricks of the professors is to have it set up facing the door to a classroom on the first day, and swing a baseball bat through the image as students walk in.

I’ve also seen the saucer devices, and I don’t know who made them, but the ones I saw were pretty convincing.