Physicists demonstrate rotated light images

(PhysOrg.com) -- In what might at first seem obvious, but isn't after further thinking, a group of physicists from the United States and Canada have demonstrated, for the first time, that images generated by light, can be rotated via a rotating medium. In a paper published in Science, physicists Sonja Franke-Arnold, Graham Gibson, Robert W. Boyd and Miles J. Padgett describe how they were able to replicate the effects of light shifting via a moving medium, in a spinning medium, opening the door to a possible new way of encoding transmitted images.

Scientists have long known that when a light is shone through certain moving material, that the light itself can be shifted along with it, due to the photons being absorbed and then released by the atoms in the medium. The effect has been demonstrated over the years and can be seen in the simplest of venues, such as light shining through a waterfall. Until now however, no one has shown that a similar effect might apply to a rotating medium.

The idea is that if a beam of light, projected in a certain shape, such as a square for example, were to be shone through a spinning medium, such as a round block of glass, the image would emerge on the other side, but not exactly opposite; it would be off, just a little bit, in the direction of the spin. The amount of shifting would of course depend on both the speed of revolution of the cylinder and on the medium used, as some, such as rubies are able to cause more of a drag, per se, on the light as it moves through, than others.

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Image rotation filmed for a light beam propagating through a 10cm length of ruby rod, spinning the rod first clockwise and then anti-clockwise (as viewed from the camera position). Video: Science, DOI: 10.1126/science.1203984

To test this theory, the team created a square beam of green light, which they then directed at a spinning cylinder made entirely of ruby. The light was sufficiently strong enough to shine all the way through the cylinder, creating a square image on the other side. To see if the image was being rotated as the cylinder spun, the exact location of the projected image was noted, then the cylinder was spun in the opposite direction, to see if it would then be in a different position; which of course, it was. With the cylinder spinning at 30 revolutions per second, they found that the projected image was rotated about a third of a degree. They also found that increasing the amount of light tended to increase the amount of rotation of the projected image, in some cases, by as much as ten degrees.

The research team note in their paper that they believe one application of this phenomenon could be its use in image encoding, just as current methods now include an image?s intensity.

More information: Rotary Photon Drag Enhanced by a Slow-Light Medium, Science 1 July 2011: Vol. 333 no. 6038 pp. 65-67 DOI: 10.1126/science.1203984

ABSTRACT

Transmission through a spinning window slightly rotates the polarization of the light, typically by a microradian. It has been predicted that the same mechanism should also rotate an image. Because this rotary photon drag has a contribution that is inversely proportional to the group velocity, the image rotation is expected to increase in a slow-light medium. Using a ruby window under conditions for coherent population oscillations, we induced an effective group index of about 1 million. The resulting rotation angle was large enough to be observed by the eye. This result shows that rotary photon drag applies to images as well as polarization. The possibility of switching between different rotation states may offer new opportunities for controlled image coding.

via PhysicsWorld

2010 PhysOrg.com

Searching for fractals may help cancer cell testing

Researchers were able to determine a cell's ability to cling onto nearby objects and mapped the adhesion points of certain cells.Credit: Victor Sokolov

Scientists have long known that healthy cells looked and behaved differently from cancer cells. For instance, the nuclei of healthy cells -- the inner part of the cells where the chromosomes are stored -- tend to have a rounder surface than the nuclei in cancerous cells.  

A new experiment looks at the shapes of healthy and cancerous cells taken from the human cervix and has attempted to quantify the geometrical differences between them. The research, carried out at Clarkson University in Potsdam, N.Y. finds that the cancerous cells show more fractal behavior than healthy cells.

Fractal is the name used for heavily indented curves or shapes that look very similar over a variety of size scales. For example, the edge of a snowflake, when observed with a microscope, has a lacelike structure that looks the same whether at the level of a millimeter, or a tenth of a millimeter, or even a thousandth of a millimeter. The position of galaxy clusters in the sky seems to be fractal. So does the snaking geometry of streams in a river valley, or the foliage of leaves on a tree. The shape of coastlines and clouds reveals a fractal, "self-similar" geometry. Even the "drip" paintings of Jackson Pollack are fractal.

Fractal geometry apparently also appears in the human body. The pattern of heartbeats over long intervals looks fractal. How about the geometry of cells? And could the observation of fractal geometry be used to identify cancer cells?

The above figure shows a cell imaged by SEM (scanning electron microscope) and AFM (atomic force microscope). Credit: Victor Sokolov

Igor Sokolov and his Clarkson colleagues used an atomic force microscope to view cells down to the level of one nanometer, or a billionth of a meter (one-millionth of a millimeter). Just as the needle on a record player rides over the groove of a rotating vinyl record to read out the music stored on the record's surface, so the sharp needle forming the heart of an atomic force microscope rides above a sample reading out the contours of matter just below at nearly atomic resolution.  

Previous studies of cells at the microscopic level produced two-dimensional maps of the cells' surface. The new study produces not only three-dimensional surface maps of geometry. But with their atomic force microscope device the Clarkson scientists can also map properties such as the rigidity of the cells at various points on its surface or a cell's adhesion, its ability to cling to a nearby object, such as the needle probe of the atomic force microscope itself.  

The Clarkson measurements show that cancerous cells feature a consistent fractal geometry, while healthy cells show some fractal properties but in an ambiguous way. The fact that the adhesive map is fractal for cancerous cells but not for healthy cells was not known before.

Being able to differentiate clearly between healthy and cancerous cells would be important step toward a definitive diagnosis of cancer. Can a fractal measurement of cells serve as such a test for malignancy?

Sokolov believes it can.  

"The existing cytological screening tests for cervical cancer, like Pap smear, and liquid-based cytology, are effective and non-invasive, but are insufficiently accurate," said Sokolov.  

These tests determine the presence of suspicious abnormal cells with sensitivity levels ranging from 80 percent all the way down to 30 percent, for an average of 47 percent.  

The fractal criterion used in the Clarkson work was 100 percent accurate in identifying the cancerous nature of 300 cells derived from 12 human subjects, Sokolov said. He intends now to undertake a much wider test.  

"We expect that the methodology based on our finding will substantially increase the accuracy of early non-invasive detection of cervical cancer using cytological tests," Sokolov said.  

The above image shows a side-by-side comparison of the adhesion for the surface of a cancer cell -- in this case, the cell attached to the needle probe of the atomic force microscope itself. Credit: Victor Sokolov

Sokolov asserts that physics-based methods, such as his atomic force microscope maps of cells, will complement or even exceed in detection ability the more traditional biochemical analysis carried out at the single cell level.

"We also plan to study how fractal behavior changes during cancerous transformation, when a normal cell turns into a fully developed malignant cell, one with a high degree of invasiveness and the ability to reproduce itself uncontrollably," Sokolov added.

Robert Austin, an expert on biological physics at Princeton University in N.J., believes it is important to learn more about the properties that make cancer cells lethal, such as their ability to metastasize, to invade new parts of the body. About the Clarkson paper, which is appearing in the journal Physical Review Letters, Austin said "Perhaps this is a step in the direction of connecting physical aspects of cancer cells with the biological reality that their proliferation and invasiveness is what makes them deadly."

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