‘It invites us to reconsider our notion of shadow’: laser beams can actually project their own shadows, scientists discover

A quirk in 3D scientific modeling led to the discovery that laser light can – under the right circumstances – cast its own shadow.

This discovery challenges our current understanding of what a shadow is, the researchers wrote in a new paper published November 14 in the journal Optical.

“Laser light casting a shadow was previously thought to be impossible, because light typically passes through other light without interacting,” study co-author. Raphaël Abrahaophysicist at Brookhaven National Laboratory, said in a statement. “Our demonstration of a very counterintuitive optical effect invites us to reconsider our notion of shadow.”

Imagine two flashlight beams crossing each other – neither blocks the other. However, some 3D computer models of laser systems represent the laser beam as a cylinder. These models give the simulated laser beam its own shadow because they treat it as a solid object rather than a ray of light. During a lunchtime conversation about this quirk of the modeling system, some scientists wondered if they could replicate the phenomenon with a real beam.

“What started as a fun discussion over lunch led to a conversation about laser physics and the nonlinear optical response of materials,” Abrahao said in the release. “From there, we decided to conduct an experiment to demonstrate the shadow of a laser beam.”

Laser beam shadow GIF.

(Image credit: Abrahao et al.)

The researchers set up an experiment with a ruby crystal, a popular material for studying the unusual properties of light. They shined a green laser and a blue laser into the ruby at right angles to each other. On a screen in front of the blue laser, they saw a dark line where the green laser blocked the blue light leaving the crystal.

This curious phenomenon comes from a property of the ruby itself. When the green laser encounters atoms on its path through the crystal, it gives the electrons in those atoms some extra energy. In their high-energy state, these electrons can absorb blue light from the second laser. The green laser beam thus behaves like an object, blocking blue light and projecting a dark line on the screen.

This dark line met all the criteria for a shadow: it was visible to the naked eye, it took the shape of an object blocking the lighting, and it moved as the researchers moved the green laser .

Scientists also discovered that they could adjust the intensity of the green laser to create a darker or lighter shadow. The maximum contrast between the bright and dark areas of the screen was around 22%, similar to the contrast of a tree’s shadow on a sunny day.

“This discovery expands our understanding of light-matter interactions and opens up new possibilities for using light in ways we had not previously considered,” Abrahao said in the release.

This discovery could prove useful in various fields, Abrahao added. An example is optical switching in devices that rely on light to control the presence of other light, or in technologies that require precise control of light transmission, such as high-power lasers.

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‘It invites us to reconsider our notion of shadow’: laser beams can actually project their own shadows, scientists discover

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