Category Archives: Video

The Aqueous Aragorn Effect

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Water Striders. Filmed in slow-motion, of course.

There’s actually a bit of physics here, starting with the obvious, the reason they don’t sink: surface tension. Water is polar, so the molecules tend to attract each other, making the surface act like a series of springs and able to support small masses, up to the point that the attraction is overpowered.

Once the strider starts moving, we can see some more physics in action:

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The water strider is actually hard to see in this picture — it’s a little above and the the left of center. The black dots are a shadow of sorts. The insect would not cast much of a visible shadow if it were on a flat surface; it’s small and light will tend to diffract around the legs. But what’s happening here is that the feet make an indentation in the water and it turns the now-curved surface of the water into a lens. And the lens is concave, so the light diverges and leaves a dark spot because light has been directed away.

The water strider has been moving, and this disturbs the water. We see the waves from this disturbance and the interference as two separate waves pass through each other.

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Once again, you’re seeing the effects displayed on the creek bed, rather than the surface.

Getting All Charged Up

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I bought another toy recently. A “fun fly stick,” which is a static electricity generator. Here are two of my colleagues playing with it. The levitating object is some aluminized mylar, i.e. tinsel.

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The low ceiling in my office makes it difficult to truly appreciate it, but does show some physics. You can see the tinsel collapse when it touches the ceiling and discharges, and then pop open again when it’s free. The charges on the aluminum repel each other, and spread out as much as they can in order to minimize their energy. That’s what we are taught in E&M, and it’s easy to see this with a deformable object rather than the canonical rigid sphere.

The tinsel does actually get charged (rather than having some induced charge distribution), which you can feel on the occasions where you get too close and it attacks your face. Not too much of a shock, though. I tried aluminum foil, but it’s too heavy. (foil = fail) Packing peanuts didn’t repel like I expected, but that may work better in drier weather. The tinsel targets are a bit delicate and I have empirical evidence that they do not stand up to the treatment of two/three year-olds.

This is an ad from ThinkGeek (not where I got this particular toy) that uses a little more free space.

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Ptole … Doh!

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Reconstruction of a planet’s bizarre orbit with Ptolemy’s system of epicycles and deferents.

What’s behind this? Any periodic function can be represented as a Fourier series. All that’s really happening here is one is plotting it in polar coordinates, and the Fourier components become epicycles. Any one-line drawing made without the pen leaving the paper and starting where it stops, should be able to be represented this way.

High-Speed Thoughts

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I’ve been having way too much fun with my new camera, and (as any regular reader knows) have posted several videos. Many more are still unedited and unuploaded (is that a word?), and even more will just sit as a collection of magnetic orientations on my hard drive because they ended up being not particularly interesting, but I don’t like throwing things out if there’s any chance they might be useful at some point.

Lessons learned, in no particular order:

Some things look interesting in slow motion, some things don’t.

Obviously, slow-moving (or nonmoving) things are a waste of a shot. If you can discern details with your eyes, slowing things down won’t help much. A fan filmed in slow-motion looks just like a fan moving at a slower setting. A medium-sized bird flying along looks a lot like a bigger bird flapping more slowly. What you need is some nonlinearity — some kind of inflection — for these situations to become interesting. A bird taking off, for instance, is a lot more interesting (to me) than one just flying along. But you often do it anyway, just to see if slowing it down reveals any details.

Some things are really frikkin’ fast, and slowing them down by a factor of 14 (30 fps to 420 fps) doesn’t really put a dent in it. The sweet spot is around 100 Hz. At 10 Hz or lower you can see what’s going on without assistance, and when you get above several hundred Hz you aren’t sampling the phenomenon often enough to truly see the detail — Nyquist’s theorem in action.

These are movies — things move

This is a no-brainer, but for someone used to taking stills, it’s a problem of adapting. Zooming in too far means that when the action happens, the target quickly moves out of frame, and it’s really hard to follow and keep the target in-frame. Zoom out too far, though, and there isn’t enough detail to make the shot worthwhile. For a regular video, I’d say pick your poison. For slow-motion, I think you can err a little on the side of zooming in, because you get more frames, and still have the chance to capture something interesting.

The flip side of this is that my motion has a smaller effect on the shot. The camera has a steady-cam setting, and any of my motion is “damped” by the frame rate. Physical jitter isn’t as annoying. I use a tripod when I can, but it’s not a must unless you need both hands for the shot itself.

Lighting is really important

Another no-brainer, but it still takes getting used to. You can’t really use a flash, and you might not realize how bright flashes are. Even though the camera films at lower resolution (and I assume this is to group pixels together to gather more light) the camera still doesn’t do as well in ambient light as a still photo. 420 fps is only about two milliseconds of exposure time, and you have to gather enough light so that noise isn’t an obstacle, which limits you. I’ve purchased a couple of LED desk lights for indoor shooting, as well as outfitting a traditional el-cheapo-desk lamp with a CFL (the fixture is limited to 60 Watts, but a 100 W-equivalent-brightness CFL draws only about 20 W). The drawback to this is that cheap desk lighting gives you a 60Hz flicker. I film near a window when I can, to get sunlight involved.

A corollary to this is that the aperture is wide open, which reduces depth-of-field. The camera has an occasional tendency to auto-focus on the wrong item, and the shot ends up being blurry. But that’s not a disaster, because …

Film is free

As with all digital photography, this is another habit one has to shake off if one is used to film and developing — the spectre of not having enough film, and letting that dictate whether you take the shot. Digital cards are cheap, and you can delete bad pictures right away if necessary. You can also decide if you want to re-shoot, (if circumstances permit) if you know that a shot was bad. If I’m doing a set-up shot with consumables, like match-lighting or balloon-busting, I prepare several samples.

Regular video isn’t all the great.

Even with the steady-cam on, there is a lot of jitter when holding the camera. I haven’t tried Rhett’s water-bottle hack to add mass, because I haven’t been in many situations where I want regular-speed video.