Tiny Drops Create Rings of Color

March 13, 2018

An Accidental Rainbow

Illuminated from above, droplets of water stuck to the underside of a glass surface are encircled by rings of color. 
Image Credit: A.E. Goodling, et al.

What's Going On Here?

A rainbow isn’t the only amazing thing that can happen when light bounces around inside water droplets! This image is a top-down view of microscopic drops of water—each about the diameter of a very fine human hair—clinging to the bottom of a pane of glass, illuminated from above.

When light enters the droplet, the curved surface of its dangling edge reflects some of the light rays back out—but not before they bounce around a few times first. As shown below, photons that reflect off the interior surface of a drop multiple times take a longer path than ones that only bounce once or twice.

Image Credit: Physics Today, American Institute of Physics.

The difference in the lengths of the paths they take is small, but that’s precisely what causes these colors to emerge! When two photons of the same color take different paths but end up heading in the same direction, they can undergo what’s called destructive interference if the peaks of one photon’s waves end up aligning precisely with the troughs of another’s.

In an individual water droplet, whether or not this happens depends on the distance between a photon’s peaks and troughs, and how that compares to the difference in path length of photons undergoing a certain number of bounces. Since the distance between a photon's peaks and troughs is also known as the wavelength—which determines light's color—a droplet of a certain size tends to "destroy" some colors more easily than others—leading to the selective reflection seen here!