Monday, July 27, 2009

Seeing the Invisible

Wow, these shadowgrams in the New York Times are amazing! They were made by Gary Settles, director of the gas dynamics laboratory at Penn State.

I really like picture 3 of the AK-47 shooting a bullet. The bullet is supersonic and has a nice double-shock cone along with a little trail of turbulance. It's clearly outrunning the spherical shockwave from the main explosion from the gun muzzle. It's also extremely interesting that there is no sound or anything else coming from any other part of the gun than the muzzle.

There's also that dark puff of gas that's moving very fast out in front, leaving it's own cone-shaped trail of turbulence behind.

I'm curious about the spherical shock wave that's a bit in front of the main spherical shock (they look like circles here of course) but only inside the bullet cone. It joins the front of the double supersonic shock where they intersect.

There's also a faint horizontal line in front of the bullet and along its path. What in the world is that? Is it some artifact from the picture making process?

Picture 4 of a revolver shooting is a great contrast to picture 3. Here the bullet is clearly subsonic, and behind the main shock. There's a shock from the back of the gun as well as from the muzzle. The things that I find fascinating here are (1) how the center of the main muzzle shock comes from in front of the muzzle and (2) how the spherical waves from the back of the gun seem to be retarded by the muzzle explosion. They are pulled back and distorted there.

Note how much comes out of non-muzzle parts of the revolver compared to the AK-47.

All of these photos (except for the last one of the insect) show how senstive light travelling through air is to disturbances. This is one of the main problems that astronomers worry about when using a telescope. Any disturbances in the atmosphere in front of the telescope (all the way to the edge of the atmosphere) and inside the telescope have similar effects. They aren't as pronounced but, then again, the high magnification involved, well, magnifies the effects. Pictures 6 and 7 of the hair dryer and candle, though dealing with unusually hot air, illustrate this well. In both picutres take note of the less extreme turbulence in other parts of the room that's still visible.

Telescope users are plagued by warm air rising from people (in front of the telescope), buildings, warm pavement, and then of course the weather. You can easily see the effects in a very similar way, but live!, if you rack a really bright star like Sirius or Vega out of focus. A hand in front of the telescope will have a smoky pattern rising from it, clearly visible, just like the effects in these pictures.

Addendum: Hah! Well, reading the attached article, Prof. Settles discusses how the images are made with a light source, curved mirror, (lens), and a razor blade. That sounds exactly like a Foucalt tester which is used when grinding and figuring a telescope mirror!