Chemists have discovered a new way of separating reflected light into rainbow colors, a phenomenon called iridescence. The surprisingly simple technique, which is a kind of hybrid of the previous ones, could have both scientific and aesthetic applications.
"It's really cool," says Kenneth Chau, an optical engineer at the University of British Columbia in Kelowna, Canada, who did not participate in the work. "I'm surprised I did not see myself in the lab."
In iridescence, an object reflects different colors at different angles, separating the white light into its constituent colors. One way to achieve this is refraction, the bending of light when it passes from one translucent medium to another. For example, a rainbow emerges when the light bends when entering the spherical raindrops, bounces on their back, then bends again as they come out of the drops. The whole process redirects different colors at slightly different angles, dispersing them to create the rainbow.
Iridescence may also occur when a thin translucent film rests on a reflective surface, such as oil on a puddle. Some light waves are reflected on the top of the film and others on the underside. Depending on the thickness of the film, its viewing angle and the wavelength of light, the waves will recombine and interfere to strengthen or cancel each other out. Such thin-layer interference gives a puddle of oil its colored stripes.
Finally, the iridescence can appear by diffraction, when the light is reflected by a more complex periodic structure, such as the grooves of a compact disc. Again, the light waves that bounce from the grooves may interfere to strengthen or cancel each other, depending on the wavelength of the light and the angle at which it is viewed. Such diffraction explains the brilliant colors of certain wings of butterflies and photonic crystals of human origin.
Lauren Zarzar, a materials chemist at Pennsylvania State University, at State College, and her colleagues report having produced a new way to iridescent. They occurred in early 2017 when they cooked spherical droplets the size of a micron containing two types of oil in which the lighter oil formed a top layer in shape lens that the researchers hoped to use as a lens. But surprisingly, when they are illuminated from above, the edges of the lenses shine in a color that depends on their size and the angle at which they are seen, the team reports today. Nature.
Zarzar says his group was certainly not the first to witness this effect. "People came up to me and said," Oh, I know exactly what you're talking about! I saw it too. Yet, a literature search did not reveal any mention of this. The researchers assumed that this had to be a refraction or diffraction effect, but these systems could not match the data, says Zarzar.
Clarity came only with computer simulations performed by Sara Nagelberg and Mathias Kolle, mechanical engineers and members of the Massachusetts Institute of Technology team in Cambridge. Their analysis showed that the irisation emerged from a new mechanism that mixed some elements of the previous ones.
In the end, the effect can be demonstrated and more 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 an edge of a droplet can bounce twice or more from the dome of the droplet before coming out near the other edge – in the same way that light is reflected on the back of a drop of water in a rainbow. However, light waves entering at slightly different distances from the center of the droplet can bounce different numbers of times. And waves that bounce at different times can interfere and strengthen, such as in diffraction or thin film. As a result, different colors appear at different angles, which can be controlled by changing the size of the droplet.
"We were really racking our brains for a while," says Zarzar. "No other explanation was up to the effect." Chau said, "They did a great job of performing experiments and detailed simulations to see how this effect would occur.
The new effect could be linked to an effect called glory sometimes observed by the passengers of planes flying above the clouds. If the sun shines just above, the shadow of the plane below will appear surrounded by a bubble. This effect is thought to result from the interference of light waves reflecting in the water droplets in the clouds.
Engineers are already using thin films and refractive particles to create iridescence in video screens, paints and decorative wallcoverings. With its simplicity and ease of adjustment, this new effect could help color the world. There is an obvious limit, says Chau: the incident white light must come from a specific direction, so the effect will not work with ambient light. "Humans are always looking for new and different ways to produce artificial color," he says. "I predict that this will certainly allow a lot of exploration."