Figure 1 – Iridescent fan, Natick, MA. (c) DE Wolf 2018.

I was struck yesterday by this giant paper fan that had been coated with a film to create and iridescent interference pattern.This is where the film has a thickness near that of the wavelength of light, such that light reflected from the top surface recombines and interferes with light reflected from the bottom surface of the film. Differences in thickness cause light maxima to occur at different wavelengths or colors, and hence the flowing pattern of color as if it is painted on.  It is much like the interference of an oil slick on water. This kind of interference is, in fact, the cause of color in butterfly wings – which we might have otherwise attributed to some sort of pigment. The image is, of course, taken with my IPhone 6.0. I am ever-ready to test its ability as a camera.