Scientists Intrigued by Discovery of Light That Can Cast a Shadow

Wait, what?

Shadow and Bone

A team of scientists has found that the narrow beam of a laser, under specific conditions, can cast a shadow — a counterintuitive finding that could open the doors for new applications of optical lasers.

Conventional knowledge presupposes that if two light beams cross, nothing of note happens. But in an experiment, as detailed in a new paper published in the journal Optica, Brookhaven National Laboratory researcher Raphael Abrahao and his team found something strange: a narrow green laser beam, when shot through a larger blue laser beam inside of a ruby crystal, created a shadow.

"Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting," said Abrahao in a statement. "Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow."

Throwing Shade

The concept for the experiment started because of a joke the researchers shared over a meal together.

"What started as a funny discussion over lunch led to a conversation on the physics of lasers and the nonlinear optical response of materials," Abrahao explained. "From there, we decided to conduct an experiment to demonstrate the shadow of a laser beam."

First, the team shot a high-power green laser through a cube-shaped ruby crystal. They then shot a different blue laser through the same material at a perpendicular angle.

Lo and behold, readings from a sensor pointed at the surface the blue laser was illuminating revealed a shadow in the shape of the green laser beam.

In simple terms, the green laser acted more like an object, not a beam of light, creating what by all accounts could be described as a shadow.

The researchers suggest this is because of "optical nonlinear absorption" occurring inside of the ruby cube. The green light increases the absorption of the blue beam, creating a darker region that casts the shadow.

The researchers measured a maximum contrast of around 22 percent, roughly the equivalent of a shadow a tree casts on a sunny day.

"This discovery expands our understanding of light-matter interactions and opens up new possibilities for utilizing light in ways we hadn’t considered before," Abrahao explained.

"This new finding could prove useful in various applications such as optical switching, devices in which light controls the presence of another light, or technologies that require precise control of light transmission, like high-power lasers," he added.

The team is now hoping to demonstrate how a laser beam's intensity can be controlled with the use of another laser, and to explore whether the same effect can be reproduced using other wavelengths and materials.

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