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If you think you know what a shadow is, think again. An international team of scientists has just demonstrated a phenomenon that could be straight out of a science fiction book: a shadow cast by light.
Researchers in Canada and the U.S. showed that in certain conditions, a laser—a narrow beam of intense light—can behave like an object by blocking other light and casting a shadow. This unexpected result, detailed in a study published November 14 in the journal Optica, could have important implications for technologies that already employ lasers, such as fiber optic cables used in HD televisions and high-speed internet. More simply, it challenges our understanding of what a shadow is.
“Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting,” Raphael A. Abrahao from Brookhaven National Laboratory said in an Optica statement. “Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow.”
Researchers showed that a laser beam can sometimes act like a solid object and cast a shadow that is visible to the naked eye. In the above image, the shadow appears as the horizontal line traversing the blue background. Credit: Abrahao et al. The experiment resulted from a lunch conversation about how some 3D diagrams treated laser beams as solid cylindrical objects and included a shadow, the researchers explained in the statement. They consequently wondered if this could be replicated in a lab.
During the experiment, the team shot a green laser into a ruby crystal, which they illuminated with a blue laser positioned perpendicular to the green one. Very simply, the high-power green laser made parts of the ruby crystal capable of absorbing more of the blue laser, casting a shadow in the shape of the green laser.
How did they know it was a shadow? It met all the familiar criteria: it was visible to the naked eye, conformed to the surface it was cast upon, and matched the position and shape of the thing that cast it (the green laser).
The team also measured the shadow’s contrast in relation to the laser beam’s power—more plainly, how stark they could make the shadow—and achieved a maximum contrast similar to the “starkness” of a tree’s shadow on a sunny day.
However, the “laser shadow effect requires a ruby to mediate” it, the scientists wrote in the study, “which raises the interesting question of whether the photons in the object laser themselves are blocking the illuminating light or whether it is the atoms in the ruby.” In other words, they were not completely certain whether it was the laser casting the shadow or the ruby.
Regardless, the demonstration expands the possibilities of controlling one laser via another. While the practical applications for this might not be immediately obvious in everyday life, it certainly prompts us to keep a closer eye on the shadows—or lack thereof—in Hollywood’s laser duels.
