Flying Circus of Physics

Flying Circus of Physics 

Newsletter for Aug - Oct 2013

Shadowplay

Here’s a challenge: Find something interesting about shadows. After all, a shadow is merely the absence of light because an object blocks the light. Ah, but the really interesting feature of a shadow is our interpretation of it, which can be magical.

Hand shadows

You saw some of that magic as a child with hand shadows we interpret the dark regions in terms of ducks and other animals. Here is a sample.

http://www.youtube.com/watch?v=EAQxNVQF_I0

Shadow dancing

In recent times, creating the illusion of objects other than simple animals has become an art form called shadow dancing. Here is my favorite example:

http://www.youtube.com/watch?v=FYftvseVzuI 

Shadow hiding

In those two videos and in your everyday life, shadows are due to the blockage of light, but they cannot themselves be blocked. Several years ago Roberto Casati (of the Institut Jean Nicod in Paris) raised a fascinating question: Is that really true? Can you possibly hide your shadow as a young child might try when placing some object over the shadow? We as adults know that is impossible. If you place, say, a sheet of paper over a shadow on the ground, then the shadow is on the paper rather than the ground. It certainly has not disappeared.

    Nevertheless, Casati discovered a way to hide a shadow. I followed his example by setting up the shadow of a pencil lying across the end of an upright cardboard tube, all in bright sunlight. What I saw on my driveway was exactly the mundane example I expected: a combined shadow of pencil and tube. I then made the shadow of the tube seemingly disappear by filling that shadow with white sand. Although the sand was in the shadow region, the grains still scattered enough ambient light to me that the region seemed relatively bright, certainly brighter than the shadow of the pencil. And to my astonishment, because of the contrast between the two shadow regions, the pencil shadow appeared to extend under the sandy region, as if it was hidden from view. If you would like to see my photographs of the shadows with and without the sand, go to the Vision part of the main Flying Circus of Physics site

http://www.flyingcircusofphysics.com/News/NewsDetail.aspx?NewsID=43

scroll down to item 7.56 at the bottom of the page.

Shadow magic

One of the most stunning magic tricks of Penn and Teller (or anyone else) is a shadow trick invented by Teller when he was a teenager. We see a rose and its shadow on a screen. As Teller touches a knife to a portion of the shadow, he seemingly cuts off  that portion and then the corresponding part of the rose falls off. Normally we say that a shadow is affected by changes in the object casting the shadow. But in this illusion, the reverse occurs: We must conclude that changes in the object are affected by changes in the shadow.

http://www.youtube.com/watch?v=etuVHEHF3FM

 Shadow reversal

The shadow of a moving object moves. Oh, there might a complicated surface on which the shadow moves, such as the corner of a building. Still, we can usually determine the motion of the object by simply watching the motion of the shadow. Here is an example in which the object (off camera) is the outline of cat that rotates around a vertical axis. We see only the rotating shadow, but which way is the rotation? Is it rotating clockwise or counterclockwise around the vertical?

http://www.youtube.com/watch?v=tDQ78MIcLYw

When I first watch the video, the shadow (and thus the off-camera cat outline) is rotating counterclockwise as would be seen from overhead. But if I blink my eyes several times, the rotation direction suddenly reverses. The rotation is ambiguous, subject to the whims of my subconscious interpretation of the motion. Here again the shadow seems to affect the object rather than the other way around.

Impossible shadow

If you look within a shadow in bright sunlight, you see a dim outline around a darker interior. The dim outline (the penumbra) is due to the finite width of the Sun in our view --- it is not a point source of light but instead occupies an angle of about 0.5 degree. That finite width means that the shadow edge is not sharp, and the transition from a brightly lit region to the dark region (the umbra) is a bit gradual. Still, inside the dark region, you see darkness. Not surprising and hardly worth your attention. The object blocked the direct light and only stray ambient light can reach the shadow’s interior.

     However, in some situations you can see a pattern of bright and dark bands at the center of a shadow. The existence of such a central pattern was of fundamental importance in developing our modern science of optics. Its discovery began in 1818 when Augustin Jean Fresnel submitted a wave model of light to a competition at the French Academy. At the time, everyone believe Isaac Newton’s long established theory that a light beam consists of particles. Simeon D. Poisson, one of the members of the competition’s judging committee, argued strongly against Fresnel’s wave model, attempting to reduce it to absurdity with this thought experiment: If an opaque object with a circular cross section (such as a coin or a ball) is illuminated with a beam of light, Fresnel’s wave model predicted that a bright spot should appear at the center of the shadow cast by the object on a distant viewing screen.

    Dominique F. Arago, another committee member, arranged to test the prediction in spite of the absurd conclusion. Surprisingly, he found the bright central spot. Through a quirk of history, the spot is now known as either the Poisson spot or the Arago spot, although neither man initially believed in its existence.

    To see my photograph of the Poisson spot produced in the shadow of an opaque circular dot by the light of a helium-neon laser, go to

http://www.flyingcircusofphysics.com/News/NewsDetail.aspx?NewsID=42

and scroll down to item 6.151.

     An explanation of the Poisson spot is also given there. In short, it is due to the wave nature of light, as modeled by Fresnel. When the light waves pass the edge of my opaque dot, they spread around the edge and into the shadow region. The light is imperceptible except near the axis through the dot’s center. There light from all around the dot’s perimeter comes together to form a bright dot with surrounding bright and dark circles --- the Poisson spot.

August 2013

The August 2013 stories at the main FCP site include the curious case of “death by bunk bed” that I discovered in one of the forensic science journals I read.

http://www.flyingcircusofphysics.com/News/NewsDetail.aspx?NewsID=5

The latest pub trick is also included.

http://www.flyingcircusofphysics.com/News/NewsDetail.aspx?NewsID=44

Again, as with all the pub tricks at the site, the real challenge is in explaining the physics that makes a trick work.

More videos and photos have been added to the FCP Facebook site:

http://www.facebook.com/pages/Flying-Circus-of-Physics/339329532602?ref=ts

Cheers,

Jearl Walker