Quételét Pattern on Pond Scum

December 16, 2015


Photographer: Douglas McGrady
Summary Authors: Douglas McGrady; Jim Foster

The photo above showing pastel colors of water impounded by beaver activity was taken near West Greenwich, Rhode Island on December 6, 2015, at about 11:00 a.m. I first noticed the colors when I was crossing a footbridge over the still water. When I looked downstream, toward the Sun, there were wide bands of faintly colored light, reminiscent of a rainbow, upon the water’s surface. However, if I looked in the other direction, away from the Sun, the water looked normal. Two hours after taking the first photos, the colors hadn't changed much. This time of year the Sun is fairly low in the sky, even at midday. Note that there was some scum on the water's surface (pollen, dust, detritus, etc.).

The midday time frame gave a clue that I wasn't observing a rainbow since the Sun, though low, was too high for a rainbow to form. In addition, rainbows are observed when looking toward the antisolar point -- not toward the Sun. Rather, this phenomenon is known as a Quételét Pattern. The pond scum is composed of micron-sized globules or particles on the water surface (or held by surface tension just above it) that diffract sunlight. Sunlight hits the particles directly and indirectly after being reflected from the water surface. The incident light is scattered, mostly in a forward direction, by the minute particles. Thus there are two different outgoing waves from the particles that overlap and combine with each other in such a way that the wave fronts are either in phase or out of phase. This interference results in the subtly colored bands.

Photo Details: Camera Make: NIKON; Camera Model: COOLPIX P80; Exposure Time: 0.01094 second; Lens F-Number: ƒ/3.5; ISO Speed Ratings: 64; Focal Length: 7.4 mm.