Tag Archives: museum

A stunning display of natural birefringence

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In a recent visit to the Penn Museum— the University of Pennsylvania Museum of Archaeology and Anthropology –we came across a most unusual artifact in their Chinese Rotunda: a giant crystal ball:

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For a higher-quality image– without the display case– take a look here.

Here is what the display placard has to say:

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Crystal Sphere
Rock crystal, Silver Stand
Qing Dynasty (1644-1911 CE)

An ornamental treasure of the Imperial palace in Beijing, the crystal sphere was said to have been a favorite possession of the Empress Dowager Cixi (1836 -1908 CE), under whose watch imperial China crumbled. The rock crystal originated in Burma and was shaped into a sphere though years of constant rotation in a semi-cylindrical container filled with emery, garnet powder, and water. The forty-nine pound flawless crystal sphere is believed to be the second largest in the world. The stand in the shape of a wave was designed by a Japanese artisan.

So, not only is it a giant crystal ball, but it’s a historically interesting giant crystal ball. But besides that– and its brief modern stint as a hat rack –what’s genuinely remarkable about this particular artifact is that it’s made from a chunk of rock crystal, better known as quartz crystal.

Now, those “crystal balls” that run-of-the-mill fortune tellers use are often just glass— glorified playground marbles or perhaps so-called lead crystal, which is actually just another type of glass.

Quartz crystal, on the other hand, has a structured atomic lattice that leads to some very interesting physical properties including piezoelectricity, triboluminescence, and birefringence. These properties arise from the crystal structure itself; they are typically minimal or absent in glasses such as fused silica (glass made by melting quartz crystal).

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While the museum probably wouldn’t want you compressing or grinding their crystal ball for piezoelectricity or triboluminescence experiments, the birefringence is boldly sitting out on display.

Let’s look a little closer:

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The sign, across the room reading “TEXTILES” is not just inverted like it would be with a spherical lens, but also– plain as day –appears as double image, even through our single camera lens.

Why? Quartz crystal is a birefringent material, which means that light rays entering the material experience two different indices of refraction, depending on their polarization and orientation with respect to the crystal lattice. In practice, our eyes see all polarizations, so this means that the crystal ball acts like a superposition of two glass balls with different indices of refraction– and light rays entering the sphere at any given point can follow two different paths to reach your eyes. Hence the double image.

It’s also worth noting that the two separate images are composed of photons with perpendicular polarization. If you were to look at this sphere through a linear polarizer (e.g., one lens of the 3D glasses that they use in modern movie theaters), you could turn it such that only one of the two images was visible at a time.

Birefringence is not particularly rare, and there are materials (like certain forms of calcite) that have huge, easily visible birefringence. Optical devices made from flawless natural calcite, exploiting this property, are tremendously important to scientific research and industry.

We tend to think of a quartz crystal as being perfectly clear– not something that gives you a double image when you look through it. That’s because quartz is only very weakly birefringent, especially when compared to calcite. Quartz is, however, still extensively used in industry in applications for which high transparency and very slight birefringence are key, such as optical wave plates. And, what’s truly remarkable about the Penn Museum sphere is that this tiny property– usually so hard to see –is so plainly visible to the human eye.

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Finally, as we mentioned, the amount of birefringence depends on the orientation of light rays with respect to the crystal itself.

This means that if we walk one quarter circle around the sphere to a point where we’re closer to looking directly along (or perhaps, perpendicular to) the optical axis of the quartz sphere, the image suddenly becomes (if you’ll pardon the pun) crystal clear.

Field trip: Marine Mammal Center

Marine Mammal Center

The Marine Mammal Center, located in Sausalito, California, is an institution dedicated to the study and health of marine mammals, particularly seals, sea lions, otters, and whales. In their extensive veterinary programs, they rescue, rehabilitate and often release many of these animals, and work to identify causes of illness and injury.

Visitors to the center can see some of the healthier patients (not the ones in the ICU) in these outdoor hospital pens shaded by solar panels as well as the research labs and a great many exhibits about these creatures.

Marine Mammal Center

We were recently invited to a behind-the-scenes tour of the center to get a first hand look at some of the amazing equipment and machinery that is needed to run a hospital for these unique patients.

In what follows, we’ll show you some of the neat things that most visitors don’t get to see, from glowing purple plasma to Nike missile silo blast doors.

Continue reading Field trip: Marine Mammal Center

Interactive Game of Life Exhibit


Recently we put together this interactive Game of Life display as an educational adjunct for a new exhibit by the San Jose Museum of Art on the works of Leo Villareal. Leo primarily works with light sculptures, and we’re very excited to see (and participate in) the exhibit, which opens this Friday.

We put together a video showing off the project, embedded here:

(If you can’t see the video here, click here to visit the youtube page.)

Continue reading Interactive Game of Life Exhibit

Evil Mad Scientist at Rods and Mods

While we don’t normally find ourselves as part of the case mod community, we’ve been invited to participate in the Rods and Mods event, “The Kustom Kulture of Radical Computer Modification” currently going on, Thursday through Saturday at the Exploratorium in San Francisco. We’ll be attending this Saturday, showing off some of our recent projects and maybe even some hacked devices.

You can read more about this event here, and we’ll hope to see you there!

Young Makers at the Exploratorium

We’re thrilled to be heading back to the Exploratorium this Saturday for the first Young Makers event put on by Make, the Exploratorium, TechShop, and Pixar.

Here’s a little bit about the program from Make’s writeup: “It’s a first experiment in a new program we call Young Makers, in which we intend to create an infrastructure to nurture kids who want to learn by making, beyond what they can do with construction kits. We hope to fulfill a dire need: satisfying a little bit of what shop classes used to do before they, lamentably, started getting booted out of schools.”

We hope to see you there this Saturday between 11 am and 3 pm. There will be BristleBot building going on, as well as BlinkyBug building with our friend Ken Murphy. Ken also has an exhibit called A History of the Sky that is at the Exploratorium through the end of the month and is definitely worth seeing. If you can’t make it this weekend, they’ve got a great line-up of folks for the last Saturday of each month leading up to Maker Faire:

February 27th: Wearables & Soft Circuitry — Adrian Freed

March 27th: Make Your Own Kind of Music — Walter Kitundu and Krys Bobrowski

April 24th: Motors & Mechanisms — Brad Prether and Ernie Fosselius

Update: Dale Dougherty posted an article about the event here.

Webcast links

We got all packed up and headed out to the Exploratorium today for the Maker Webcast about 3D Fabrication in sugar. Thanks to Dale Dougherty from Make Magazine for inviting us – we had a great time! In case you missed it, here are the links to the archived show on the Exploratorium website: mp4, real player, windows media. (If in doubt, try the mp4 version!)

CandyFab at the Exploratorium Maker Webcast

CandyFab SignCome see the CandyFab 4000 in action on Saturday, July 28 at 1:00 PM (PDT) at the Exploratorium in San Francisco or live online for the Maker Saturday Webcasts.

Make: Magazine is partnering with the Exploratorium to bring weekly webcasts of interviews with makers this summer. If you haven’t been to the Exploratorium yet, use this as an excuse! It is one of our favorite sources of inspiration.

DIY exhibit in Chicago


We’re infiltrating Chicago! More accurately, we’re sending a few of our projects there for a little while. In the box are: the RC Sweeper, Shuffle Headphones, a holiday LED ornament and menorah, and ornament and menorah kits.

They are heading to an exhibit on DIY called “Pass It On! Connecting Contemporary Do-It-Yourself Culture” at the A+D gallery, which is affiliated with the Department of Art and Design at Columbia College Chicago.

This exhibit is unique in that all of the items on display will have accompanying instructions for viewers to take home. There will be some incredibly creative folks participating in the exhibit, so if you’re going to be in Chicago this spring (March 1 to April 14) be sure to check it out.

Thanks to the A+D folks for inviting us!

Homopolar motor exhibit at the Exploratorium

A couple of weeks ago, we visited the The Exploratorium, where we saw this giant homopolar motor, labeled “Daisy Dyno.” This is a classic demonstration of a homopolar motor. There’s a giant permanent magnet. In its jaws sits a big copper disk that is free to spin. A low-voltage but high current power supply is provided, where the positive end is hooked to the bearing in the center of the copper disk.

To run the motor, you touch the loose lead from the negative end of the power supply to the edge of the copper disk. A neat little arrow shows you exactly where to touch. The electric current flows from the center of the disk to that point of contact, which is in the direction perpendicular to the magnetic field, which creates a force in the correct direction to cause the disk to spin. In order to help the electric currents move in a fairly straight line between the edge of the disk to the center, the disk has a lot of radial slits cut through it, giving the disk the appearance of a daisy (hence the name) when it’s at rest. In the photo here, it’s moving pretty quickly.

We, of course, are quite fond of motors and magnets and things that spin, and have (so far) written up three under-one-minute science projects that are related: How to build a homopolar motor, how to make the version that spins water instead of a metal disk, and how to make a super-simple directional compass.