Diffraction and Refraction in a Spider Web

October 21, 2015


The picture above shows a spider web purposely photographed out-of-focus to capture the iridescent colors (at center) caused by the diffraction of sunlight on its fine, silken threads. Curiously and accidentally, a drop of water on the same web (at upper left-center) created a decomposed image of the familiar rainbow colors -- an example of refraction and reflection of sunlight.
The iridescent colors result from interference of light interacting with arrays of tiny, sticky droplets on the web threads. For minute water droplets, or similarly sized threads, wave interference (light behaves both as a wave and a particle) broadens the dispersed sunlight causing the colors to overlap. With refraction, a ray of sunlight passing from the air to a water drop suffers a deviation that's different for each electromagnetic wavelength. But since there's little overlap with larger raindrops, the colors are purer than the more metallic colors associated with diffraction of sunlight.
If you examine this single drop closely, you'll notice that the colors fall off in intensity more abruptly with the red and yellow colors than with the blues and violets -- the red color is sharper than the blue color. This is also the case with a rainbow. Photo taken in Curitiba, Brazil, on May 6, 2015.