A new experiment has demonstrated something that sounds physically impossible – light itself casting a shadow. It turns out that if you manipulate a laser just right, then hit it side-on with another light source, it’s possible to create this bizarre optical effect.
Shadows are so familiar in everyday life that it feels a little weird having to define them, but they appear as a darker area on a surface, where an object has blocked incoming light. The key word there is actually “object” – whether it’s a solid, liquid or gas, shadows are only known to be made by objects with mass.
That should rule out light, since photons are massless. But in the new study, researchers at Brookhaven National Laboratory have found a way to, paradoxically, make light cast its own shadow.
“Laser light casting a shadow was previously thought impossible since light usually passes through other light without interacting,” said Raphael Abrahao, lead author of the study. “Our demonstration of a very counter-intuitive optical effect invites us to reconsider our notion of shadow.”
To create this laser shadow show, the team shone a high-powered green laser through a cube made of ruby. This was then lit up side-on by a blue laser. The green light ends up blocking some of the blue light, casting a shadow on a screen with a maximum contrast of about 22% – equivalent to the shadow a tree casts on a sunny day, the team says.
R. A. Abrahao, H. P. N. Morin, J. T. R. Pagé, A. Safari, R. W. Boyd, J. S. Lundeen
The effect works thanks to some intriguing optical physics. Where the green laser hits the ruby, it increases the amount of blue light the crystal absorbs. When that blue light then hits the screen behind it, it leaves a slightly darker outline in the exact shape and position as the green laser.
“The shadow we observe is not some clever scientific analogy, nor a subtle technical effect, but rather a genuine shadow that has the usual characteristics of a typical shadow created by a material object,” the scientists write. “We observe a regular shadow in the sense it can be seen by the naked eye, it follows the contours of the surface it falls on, and it follows the position and shape of the object (the laser beam).”
The experiment itself was more of an exercise in investigating whether an intriguing idea was possible, rather than something that might have direct applications in the real world. But still, the team suggests it could lead to new possibilities for fabrication, imaging and illumination.
The research was published in the journal Optica.
Source: Optica via Eurekalert
