Tuesday, February 20, 2018

The Mirascope

I saw my first mirascope upon my initial visit to the Exploratorium when I was eight. I can still recall my astonishment. The exhibit entices you to pick up an illuminated spring, about four centimeters high and two centimeters in diameter— yet when you reached out to touch it, the spring is simply just not there! This was some serious physics magic! Sometimes mistaken for a hologram; this image is not produced using a LASER and the physics of interference and diffraction, but instead produced only by mirrors and reflection.

Here is a demonstration of a mirascope:

The physics of the mirascope is fairly simple and yet the resulting 3D ghostly image seems simply magical. The mirascope consists of two parabolic mirrors facing each other in a clamshell fashion. The key to the design is that the focal point of each parabolic mirror sits at the vertex of the other, and a hole is made in the top mirror's vertex where the image is produced. To understand how reflection of light can create such an image, consider the special geometry of a parabola. A ray diagram illustrates how parallel rays of light that reflect off a parabolic curve will all meet at its focus (figure 1). This is the operating principle of satellite dishes or any parabolic reflector. It of course works in reverse: a light source located at the focus will reflect off the curve and leave the dish as parallel rays, a phenomena used by microwave communication antennas and searchlight reflectors.

figure 1: Parallel rays reflect and converge at the focal point F
Placing two parabolic mirrors into the mirascope configuration (as seen in figure 2) puts the object to be viewed at one focus. The light leaving the object at this focal point reflects off the top mirror into parallel rays directed down. These rays then hit the bottom mirror which reflects them a second time to converge at the top focal point, creating the image at the top.

figure 2: Diagram included on the packaging
of the Opti-Gone Mirage
Amazingly, the mirascope was discovered by accident. Here’s a brief summary of the account (as described in these student conference proceedings by Adhya and NoĆ©, page 367).  Sometime around 1969 a custodial worker at UC Santa Barbara was cleaning out a storage closet in the physics department. The closet contained a collection of carefully stacked WWII surplus searchlight reflectors— parabolic mirrors, each with a hole in its center for an arc lamp to protrude through. Serendipitously, these reflectors were stacked and stored in a clamshell fashion. The worker, Caliste Landry, found that there was dust “floating” in air at the top hole of one of the reflectors that “could not be cleaned”. He reported what he found to one of the young physics faculty members, Virgil Elings, who figured out the physics of the situation. Elings and Landry were awarded a patent two years later for their “Optical Display Device”. The rights of the patent were acquired by Opti-Gone International in 1977, and to this day Opti-Gone is the main seller of mirascopes which they market under the name “Mirage”. Elings went on to found Digital Instruments Inc. in the 1990s, where he attained many patents on scanning probe microscopy—  however, the mirascope patent, a physics toy, was his first!

Get one here!
The larger version (Diameter = 9 in) as seen in the above video:

From Educational Innovations: BUY NOW "Mirage" Mirascope
From Amazon: BUY NOW "Mirage" Mirascope

A smaller version (Diameter = 6 in) that works well too:
plastic frog included =)
From Educational Innovations: BUY NOW Small Mirascope
From Amazon: BUY NOW Small Mirascope

Friday, September 16, 2016

the Swinging Sticks Desktop Toy

Special offer for @physicsfun followers:
free worldwide shipping

The craftsmanship of this product is superb- the best kinetic sculpture of its kind available- and only $98.00 US.
@GeelongShopcom has kindly offered to share a special coupon code with all of my Instagram followers. The code given below will provide free worldwide shipping on any purchases of The Swinging Sticks Desktop Toy version from their website:
From GeelongShop.com: The Swinging Sticks Desktop Toy
Use this Coupon code: FUNSHIPPING

The Physics:

Made famous when featured on the desk of Pepper Potts in Iron Man II, this sculpture displays chaotic motion that is both pleasing and memorizing. Based on the physics of the coupled physical pendulum, the motion of the rods is extremely sensitive to the initial conditions upon when they were set into motion, the essence of the technical term chaotic motion. The intricate motion is maximized by careful consideration of the location of the pivot joints, and by weighting the rods just right.

The rods are kept in motion by an electromagnetic kicker circuit in the base that pushes on a magnet in the tip of the longer rod. The essence of a basic kicker circuit is quite simple and includes the following components: a coil which serves as an electromagnet, 4 AA batteries to energize the coil, and a transistor as a switch. I have not disassembled the base of my sculpture, but I'm quite sure the circuitry is similar (and more advanced) to some of my other physics toys such as the Mystery Top and the Space Wheel that use the design described in this 1974 patent: Novelty Electric Motor.

This kind of kicker circuit is very efficient with no moving parts (except for the sculpture itself). This efficient use of electricity, along with careful engineering with respect to balance and minimal friction, leads to a battery life of up to 2 years.

Swinging Sticks Desktop Toy in action:
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The Swinging Sticks Desktop Toy: a desktop version of the famous Ironman double pendulum for just $98 US. Use this code to get free world wide shipping (coupon code = FUNSHIPPING) on all purchases of The Swinging Sticks Desktop Toy on @Geelongshopcom. This kinetic art device is the ultimate physics toy, and was featured in the movie Iron Man II (sitting on the desk of Pepper Potts). The physics magic of this device displays the essence of chaotic motion with coupled pendulums- the placement of the rods' axes of rotation are engineered to exhibit the widest range of interesting motion. Four AA batteries and a simple kicker circuit in the base gives a push to a magnet in the end of the large rod keeping the system in motion for over a year! ➡️ Follow the link in my profile for info on where to buy this and many other amazing items featured here on @physicsfun #physics #physicstoy #pendulum #coupledpendulums #coupledharmonicoscillator #harmonic #physicalpendulum #chaos #chaostheory #nonlinear #nonlineardynamics #strangeattractor #science #kinetic #kineticart #swingingsticks #theswingingsticks #ironman #pepperpotts #scienceisawesome

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Tuesday, January 20, 2015

Welcome IG friends

Welcome to my orphaned blog! I hope to resume occasional posts in the coming months.

Since October of 2013 I have been posting about 5 IG posts per week as a means of showcasing my collection of physics toys and other enigmatic objects of science. My posts are primarily videos since that format allows the fullest exploration of the kind of phenomena exhibited by the objects I collect. My initial goal was to post a more detailed blog entry here for each one of my IG posts, but I underestimated the time it takes in composing these blog entries and found it challenging to do better than what can be found on the Wikipedia pages that exist for the most famous of these science toys. Instead I discovered that composing a meaningful video of physics phenomena within the strict IG limit of 15 seconds is tricky yet tremendously fun and rewarding.

Thus far I have not run out of material to post and I hope to continue posting on IG for the foreseeable future-- and I will do my best to showcase an item per month in more detail here.

Many have asked me where they can buy these amazing items-- check out my Physics Toy Store page for my recommendations and sources. Thanks for reading!

Friday, November 8, 2013

the radiometer

My fascination with the radiometer began in the gift shop of the Griffith Observatory after an exciting day with my parents and siblings− a day that included a showcase Foucault pendulum swinging in a large pit and a giant crackling tesla coil spewing lightning.

On a glass shelf in a well-lit display case near the cashier I spied what looked like a light bulb on a stand. A 60 watt desk lamp was in place to illuminate the glass orb, and inside the orb something reminiscent of a small weathervane was spinning like crazy. I stood mesmerized. Somehow my parents bought one (rarely where such requests granted) and for years it sat in the window of our dining room.

Here is a radiometer in action:

A video posted by physicsfun (@physicsfun) on

Invented by William Crookes, a brilliant chemist and discoverer of the element Thallium, the core of the device is a balanced assembly of four small squares each painted black on one side and silver on the other, and mounted on a glass spindle. This assembly is called the fly. The fly is placed on a needle point which serves as a bearing that provides minimal friction. The tricky part of the construction is that the bulb needs to be almost (but not completely) evacuated of air to make a functioning radiometer. The was hard to do in 1876, yet the technology to do so, a Geissler mercury vacuum, was just invented a couple years earlier and Crookes had one in his lab.

As one can see from the video, the fly within the radiometer spins with the silver sides leading and the black sides following. Many great names in physics have worked on a credible physical explanation for why the fly spins at all, and the radiometer was at the center of the debate of whether light is a wave phenomenon or could be described as small particles. (We now know the answer is both, but that is another story!) The famous physicist connected to fluid dynamics, Osborne Reynolds (of the eponymous Reynold’s number) came up with the best explanation in 1879. The light energy is absorbed by the fly and its panels heat up. The fly is then propelled by gas moving from the hotter black side to the cooler white side of each panel. More than 40 years later, none other than Albert Einstein showed that Reynold’s effect was not enough for the measured speed of rotation, and in a paper published in 1924 (only available in German it seems) gave further details of how thermomolecular flow near the edges of the panel contribute significantly to the generation of rotation.

The physics of the radiometer continues to be detailed and refined in a number of recent papers. A 2010 Applied Physics Letters paper by Hin et al discuss how a micro-motor based on the Crooke's radiometer might be used to power a rotating sensor in live heart tissue, where the rotation is powered by light from a fiber optic cable instead of an electric current (which can disrupt heart function).

Yet another physics toy with a long history, able to garner the attention of many famous physicists, and remaining the subject of scientific curiosity and perhaps even life saving application.

A note of caution: Most radiometers found in museum gift shops (and in many online catalogs) have packaging that claims the device shows the presence of radiation pressure− while it has been long shown that light radiation does exert pressure, radiometers do not in fact operate on this principle!

Available from these sources:

From Educational Innovations: BUY NOW Radiometer

A wide variety available here, including some nice blown glass displays:

From Amazon: BUY NOW Radiometers

Hundreds of options on eBay:

From eBay: BUY NOW Radiometers

Tuesday, September 24, 2013

the Levitron

Few phenomena capture our attention as does the act of levitation— so counter to our expectations that gravity has cemented within our minds, we mostly find it on the magician’s stage. Yet here it is, the Levitron: spin stabilized magnetic levitation with no batteries or power source, manufactured by the cool gadget company Fascinations. The Levitron has two main components, a large donut shaped permanent magnet in the base, and a disk shaped magnet in the top itself, which are oriented such that the like poles of these two magnets repel. Hence, the pull of gravity is balanced by a magnetic repulsion, allowing the top to float for minutes at time.

Here is my short video on the operation of the Levitron:

To any physicist, an acute astonishment is felt upon a first encounter with this toy; it seems to violate Earnshaw’s Theorem which states that no configuration of non-moving permanent magnets can be in equilibrium. Typically if one tries to float a magnet above another, the loose magnet will quickly flip over and the opposite poles will come together with a snap. The key to achieving equilibrium (magnetic force v. gravity) with the Levitron is that the top is not stationary, it is spinning. Just as the conservation of angular momentum fixes the direction of a spinning gyroscope’s axis of rotation, the spinning top is similarly stabilized— but that’s not the complete story as other subtle physics principles play a role.

The actual act of getting the top to float is quite challenging. The strength of any permanent magnet is sensitive to temperature, so the repulsion force between the top and base can change from hour to hour, or from place to place if the Levitron is moved. The mass of the top must be adjusted precisely such that the push between the two magnets exactly balances with the pull of gravity— the smallest weight, an O-ring with a mass less than 1 gram, can make the difference of levitating or not.

In addition the axis of the top must tilt at a slight angle to become trapped, and the top must be spinning above a certain rate. Surprisingly, if the top is spinning too fast it will not float! These operating parameters of the Levitron, and many other surprising details, are described in this recommended paper by Martin Simon of UCLA. A thorough, high-level treatment of Levitron physics is presented in this seminal paper by Sir Michael Berry, where he shows that the magnetic trapping of a Levitron top is analogous to that used to trap single neutrons.

There are about five models of Levitron on the market. My favorite is the cherry wood model in the video, which allows a flight altitude of about 10 cm. No strings or illusions- just physics!

This physics toy is not currently in production, but some models are still available:

From eBay: Levitron Top

Wednesday, September 4, 2013

the tippe-top

Perhaps the most famous of physics toys: the tippe-top. Give this mushroom shaped top a spin on a semi-smooth surface and it will not only invert but change spin direction as it jumps onto its stem.

Here is a precision tippe-top made of aluminum in action:

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The dynamics of this top's behavior has been analyzed in numerous scientific papers, perhaps most famously in this paper by MIT physicist Richard Cohen published in 1974.  The inversion is primarily due to a torque applied to the top from slipping friction at the top's point of contact with the table. Torque, applied to existing angular momentum, leads to this dramatic inversion of the top's spin axis. The essence of Cohen's analysis is wonderfully captured in this short description and diagram by Frans Bilsen (caution- applied vector analysis involved).

No physicist can resist the allure of this physics toy, as this photo of two Nobel Laureates "demonstrating" the tippe-top will attest!

Figure: Wolfgang Pauli and Niels Bohr demonstrate a 'tippe top' toy at the inauguration of the new Institute of Physics at Lund, Sweden (1954) 
Photograph by Erik Gustafson, courtesy AIP Emilio Segre Visual Archives, Margrethe Bohr Collection

Educational Innovations has reasonably priced wood tippe-tops in their shop:

From Educational Innovations: BUY NOW Tippe-Tops
From Amazon: BUY NOW Tippe-Tops

the rattleback

Few toys have so wrought the attention of physicists as the rattleback. Attempts to pin down the physics behind the curious motion of this device are woven throughout the peer reviewed literature, and the sophistication of the math in some recent articles is on par with serious rocket science (as in this article by Lasse Franti of the University of Helsinki).

Here is a rattleback I crafted from a piece of walnut with some sand paper and a bit of effort:

A video posted by physicsfun (@physicsfun) on

Rattlebacks all have a curious preference for a particular direction of spin− this one's "direction of stability" is counter-clockwise. Note that the reversal has two parts: First the clockwise spin is reduced as the kinetic energy transfers to a wobbling motion, then the energy transfers from this wobble to a spinning motion in the final direction.

The change of direction arises from a complex interplay of friction between the rattleback and the table top, a dance enabled by an instability due to the preferred axis of rotation not being aligned with the geometric axis of symmetry. This is quite like an unbalanced tire; vibrations will occur when spun. Early in the video one can see that the bottom of this kind of rattleback is somewhat propeller shaped. This asymmetry allows the preferred axis of rotation, related to the distribution of mass within the rattleback, to be different than the geometric axis. An alternative design has instead a symmetric bottom, but with added weights to offset the preferred rotation axis (of which I will show an example in a future post).

A shape so simple, yet behavior so complex that the most advanced theoretical techniques are needed to model it. A classic physics toy!

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Acrylic Rattleback: a favorite from my collection- prefers to spin counter clockwise. If spun clockwise, a complicated combination of friction, precession, and instability induced vibrations transforms the rotational energy into into rattling (energy of oscillations) and then into rotational energy in the opposite direction! This behavior is related to the asymmetric shape of the bottom of this kind of rattleback, it's somewhat propeller shaped with an "S" curve along the bottom ridge. ➡️ Follow the link in my @physicsfun profile for info on where to get a rattleback and start your own collection of science toy. #rattleback #celt #physics #physicstoy #rotation #instability #kineticenergy #torque #friction #angularmomentum #precession #science #scienceisawesome #kineticart

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These acrylic versions work great and are inexpensive:

From Educational Innovations: BUY NOW Rattlebacks

From Amazon: BUY NOW Rattlebacks

From eBay: BUY NOW Rattleback