Category Archives: Experiments

Blame it on Eddy

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“Eddies,” said Ford, “in the space-time continuum.”
“Ah,” nodded Arthur, “is he. Is he.”

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Everyday Electromagnetism

This time, though, Eddies in the penny. And he enforces Lenz’s law.

You can see a similar effect if you drop a magnet down a copper pipe, because the eddy currents will flow, and the induced field is such that it opposes the acceleration, so you get braking.

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If you want to be more practical, instead of moving the magnet you could move the copper around, cyclically, and tap into the current that would flow. Just a thought.

Is it a Spice Rack?

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No, it’s a DIY Michelson interferometer by the Celtic Mad Scientist.

In a standard Michelson interferometer, the beamsplitter would actually go at 90º to the shown orientation, so that each beam hits a mirror, but it’s all good. If your light is polarized, you’ll want to make sure that’s vertical, lest you be near Brewster’s angle when you bounce off the beam splitter.

Homer says, “Safen up! Do not look into laser with remaining eye”

Don’t end up like the Russian Ravers who were injured when the organizers erected a tent and used the lasers “indoors.”

Did I Read That Right?

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This is the kind of post I start reading, and I begin to furrow my eyebrows as phrases and sentences pop up that don’t seem right or are obviously wrong. I though it was just bad science journalism, but realized it’s a rant-y agenda piece, with the supposed “science” reporting as a setup.

Superfluids, BECs and Bosenovas: The Ultimate Experiment

It starts off OK, giving some history, but then there was

Bosons are force carriers like photons of light and fermions are the matter we can touch.

Force carriers are bosons, but not all bosons are force carriers (universal affirmatives can only be partially converted, quoth the logician) — you can construct bosonic systems from an even number of fermions. Bosons have integral spin angular momentum, and fermions have half-integral spin, and the statistics that describe their behavior is different. An attempt to bridge the gap between science and a lay explanation that fails because it’s scientifically incorrect.

[helium is] produced by nuclear decay, as from radium and polonium, dangerous alpha radiation releasing, in fact bare nuclei of helium that eventually pick up electrons and form stable helium isotopes.

Here’s a journalistic archaeologism (it’s certainly not neo-) dangerous radiation. Nuclear radiation in invariably dangerous. Actually alpha radiation is pretty much harmless as an external dose, as it deposits its energy in a very short distance, so it doesn’t tend to penetrate even a layer of dead skin. The source is dangerous when ingested or inhaled. But the Helium nucleus is already stable (it doesn’t decay) even before it picks up the electrons — that makes it electrically neutral, not stable.

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Little Infrared Riding Hood

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My, what bright, glowing optical fibers you have.

One of my online compatriots recently explained a quick and easy way to do some IR photography. I felt compelled to try, and it was pretty easy. Cheap webcams are the most direct way to do this for a few reasons:

— they’re cheap. If you mess it up, you’re only out a few simoleans.

— they have manual focus. Modifying an autofocus camera requires you replace the IR filter with a glass plate, because removing it changes the optical path length. It’s a much trickier operation.

— it’s usually a fast modification

Just remove the lens — some of them simply unscrew — and check to see if the filter is mounted on the back. (If not, you’ll have to take the assembly apart. No biggie, though, it’s likely just one or two screws. You’ll need a jeweler’s screwdriver, probably phillips-head). Pop the filter off with a small screwdriver or equivalent; the filter may not survive in one piece, so don’t go into this expecting it to survive. Reassemble. You’re done. If the filter isn’t there, it’ll be covering the CCD/CMOS chip, but my extensive data (three points) says that it’s mounted on the back of the lens.

Plug it in to your computer and start taking pictures.

Expectations: This isn’t thermal imaging, so don’t expect bodies to show up glowing. Silicon, the element of choice, has a pretty sharp cutoff starting at about 950 nm, so what you’ll see in the near-IR. Something would have to be about 3000 K to be peaked at that wavelength and thermal images of body temperature targets peak between 9 and 10 microns. Also, the images will be small, since cheap webcams generally run only about a megapixel.

I just happen to have access to several infrared lasers (852 nm and 780 nm, the images use the latter), to give extreme examples of what you can see. This first picture is a laser table with the room lights off. You can see scattered light from several optical components, as well as light emanating from two optical fibers — not all of the light gets coupled into the fibers, and you’re seeing some of what leaks out (some probably in the wrong mode, since these are single-mode fibers, and the bending probably contributes)

IR laser table photo

In this second photo, there are two images of the same scene, taken with the room lights on. On the left, some shutters are shut, and on the right they are open, and you can see two fibers lit up. Also note the cylinder to the left — that’s a vapor cell with rubidium gas in it, set up for spectroscopy for servo-locking the laser. The laser is on resonance, so you can see the fluorescence as the beam passes through it.

As you can see, there’s quite a lot of scattered light, so normally this is encased in opaque plexiglass. None of the bright features shown are visible with the naked eye.

Photochrome

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Photochrome
You give us those nice bright colors
You give us the greens of chemistry
Makes you think all the world’s a funky lab, oh yeah!

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Zap the molecule with UV and it turns green. This is due (as I understand it) to the molecule changing to another state (isomer) — and not simply fluorescence — where it then has a different absorption spectrum, so in this example it looks green. When you remove the UV, it reverts to the original state and becomes clear again, and it’s doing this quite rapidly.

Ghostly Visages

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No, not of an alien at the window.

Here’s a little movie showing atoms being trapped and mistreated. What you’re seeing is a video of the monitor that’s hooked up to a little IR camera on the vacuum chamber. The really bright spot that’s squirming around a little are the atoms, or technically, the fluorescence from the atoms. There are probably more than a Sagan of them (i.e. biilliyuns) at about a milliKelvin or so in temperature (which is considered warm!) because it hasn’t been fine-tuned yet. The bluish circle is reflected light off of a flange, and there’s some other scattered light visible.

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About 10 seconds in, the trap’s magnetic field is turned off, and the atoms squirt off to the side. A second or two later the field is turned on again, and the trap fills up. If the lasers were properly aligned and balanced, and the earth’s magnetic field were either shielded or zeroed out with trim coils, then what you would see is a nice uniform expansion into a much colder (microKelvin-ish) optical molasses, but the earth’s field is still present here, so that gives rise to an imbalanced residual force which is small compared to the trapping force, so you only really notice it when the trapping field is turned off. The molasses impedes the motion of the atoms, but doesn’t technically trap them, i.e. it doesn’t define a point where the atoms should be, so when only the lasers are there, the atoms would normally just drift through, very slowly. But here they’re being shoved a little bit.

The accelerations involved here are large — these atoms can scatter a million photons a second, give or take, depending on the exact laser frequency, so even though an individual scatter changes the atom’s speed by about 6 mm/sec, when you scatter them that rapidly you can get accelerations of hundreds of g’s. But in the situation here, where the atoms are almost at rest, that’s balanced by an equally large acceleration from an opposing laser.

The Photon Push-Me Pull-You

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A few weeks ago, over at Built on facts, I threw Matt a bit of a knuckleball in the comments.

[C]onsider a solid bar of the same index [as water]. You send in the pulse of light (assume a really good AR coating so there’s no reflection). What happens to the speed of the bar?

This was sneaky because it is one of the unsolved issues in physics (I feel no remorse for doing this, and Matt realized that something was up) — the theory is complicated enough that it’s really easy to miss out on some of the subtleties and end up with an invalid answer. There are two schools of thought: Minkowski, who had taken the approach that the photon’s momentum in the medium should be nE/c, and Abraham, whose approach gave the momentum as E/nc. Clearly, the results are at odds, and this came to be known as the Minkowski-Abraham momentum controversy.

I found a number of articles on the topic, but perhaps the best one is a review article from Reviews of Modern Physics. Momentum of an electromagnetic wave in dielectric media by Pfeifer et. al, No. 4, October–December 2007 pp. 1197-1216. (link is to a pdf file) The article points out that this isn’t a simple problem, because a photon in a medium can’t be naively treated as just a photon — both solutions have merit, but must include the interactions with the medium, which are obviously different depending on the approach you take — in the end there can be only one you can only have one answer for the momentum of the system.
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