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Scientists luck upon a new way to make a rainbow

Chemists have stumbled across a new way to separate reflected light into the colors of the rainbow—a phenomenon known as iridescence. The surprisingly simple technique, which is something of a hybrid of previously known ones, could have applications both scientific and aesthetic.

“It’s really cool,” says Kenneth Chau, an optical engineer at the University of British Columbia in Kelowna, Canada, who was not involved in the work. “I’m surprised I didn’t see it in the lab myself.”

In iridescence, an object reflects different colors at different angles, separating white light into its constituent colors. One way to achieve it is through refraction, the bending of light as it passes from one translucent medium to another. For example, a rainbow emerges when light bends as it enters spherical raindrops, bounces off the back of them, and then bends again as it exits the drops. The entire process redirects different colors at slightly different angles, spreading them to create the rainbow.

Iridescence can also arise when a thin translucent film lies atop a reflective surface, like oil on a puddle. Some light waves reflect off the top of the film and some from the bottom. Depending on the thickness of the film, the angle at which it’s viewed, and the wavelength of the light, the waves will recombine and interfere to either reinforce each other or cancel each other out. Such thin-film interference gives an oily puddle its colorful stripes.

Finally, iridescence can arise through diffraction, when light reflects off a more complicated periodic structure, such as the grooves in a compact disk. Again, the light waves rebounding from the grooves can interfere to reinforce or cancel one another, depending on the wavelength of the light and the angle at which it is viewed. Such diffraction explains the brilliant colors of some butterfly wings and humanmade photonic crystals.

Now, Lauren Zarzar, a materials chemist at Pennsylvania State University in State College, and colleagues report producing iridescence in a new way. They happened across the effect in early 2017, when they cooked up micron-size spherical droplets containing two types of oil in which the lighter oil formed a lentil-shaped upper layer the researchers hoped to use as a lens. But surprisingly, when illuminated from above, the edges of the lentils glowed with a color that depended on their size and the angle at which they were viewed, the team reports today in Nature.

Zarzar says her group certainly wasn’t the first to witness the effect. “People have come up to me and said, ‘Oh, I know exactly what you’re talking about! I’ve seen it, too.’” Yet, a literature search revealed no mention of it. Researchers assumed it must be a refraction or diffraction effect, but those schemes couldn’t fit the data, Zarzar says.

Clarity came only with the computer simulations performed by Sara Nagelberg and Mathias Kolle, mechanical engineers and team members at the Massachusetts Institute of Technology in Cambridge. Their analysis showed the iridescence emerges through a new mechanism that blends certain elements of the previously known ones.

In the end, the effect can be demonstrated and most easily explained in a much simpler system: water droplets that condense and hang from the underside of the lid of a petri dish. Light waves entering near one edge of a droplet can bounce two or more times off the dome of the droplet before emerging near the other edge—much as light reflects off the back of a raindrop in a rainbow. However, the light waves entering at slightly different distances from the center of the droplet can bounce different numbers of times. And waves bouncing different numbers of times can interfere and reinforce each other, as in diffraction or thin-film interference. As a result, different colors emerge at different angles, which can be controlled by changing the size of the droplet.

“We were really racking our brains for quite some time,” Zarzar says. “No other explanation came close to matching the effect.” Chau says, “They did a great job doing detailed experiments and simulations to see how the effect arises.”

The new effect could be related to one called a glory that is sometimes seen by airplane passengers flying over clouds. If the sun shines from directly above, the plane’s shadow below will appear surrounded by rainbowlike bullseye. That effect is thought to arise from the interference of light waves reflecting within water droplets in the clouds.

Engineers already use thin films and refractive particles to create iridescence in video displays, paints, and decorative wall coverings. With its simplicity and adjustability, the new effect could open ways to color the world. It has one obvious limitation, Chau says: The incident white light has to come from a specific direction, so the effect won’t work in ambient light. Still, “Humans are always looking for new and different ways to produce artificial color,” he says. “I foresee that this will definitely allow for a lot of exploration.”