Smoke rings are being seen in a new light. In this case, they’re actually smoke-free.
The structures are examples of vortex rings. These are doughnut-shaped swirls that sometimes flow through liquids and gases. Erupting volcanoes can spit smoke rings into the air. Air cannons can blow smoke rings, too. Similarly, dolphins can blow rings of bubbles through water. Now, scientists can create this type of ring — with light.
Normally, a vortex is a swirl in a liquid or gas. A tornado is a one example. So is a whirlpool.
To make a vortex ring, imagine stretching out a whirlpool. That would make a long, thin swirling tube. Now, picture bending it into a circle. Then, attach the circle end-to-end. Now you’ve got a vortex ring. These rings travel through a liquid or gas as they swirl.
In the new vortex rings, light does the same thing. The flow of the light’s energy swirls. It takes the same pattern as smoke in a smoke ring. Researchers shared their discovery June 2 in Nature Photonics.
Optics researcher Qiwen Zhan led the study. He works in China at the University of Shanghai for Science and Technology. His group started with a vortex tube of light. That’s a hurricane-like structure. They already knew how to create it using lasers. The team shone the light through a special device to bend the tube into a circle. That made the vortex ring.
Light rings aren’t that different from smoke or bubble rings, Zhan says. “That’s kind of cool.” He wonders if scientists could create vortex rings out of other things. For instance, maybe they could make vortex rings out of magnetic fields.
Such rings can be described by a field of math called topology. That’s the study of doughnuts, knots and similar shapes. Studying the light rings might reveal how topology affects light and how light interacts with matter.