As we collectively face life in the middle of a deadly COVID-19 virus pandemic, some of us spent the first couple months of this virus cooped up inside our homes. Our shelter-in-place meant shelter inside because of the inclement weather outside, as we eagerly awaited signs of spring and the opportunity to be outside in the sunlight. Thankfully color is returning to the landscape as birds return, the grass greens, and bulbs and fruit trees flower. As I’m soaking in the loveliness of a particular spring day, the activity at my backyard bird feeder draws my eye. Many of the birds’ feathers are iridescent in the sunlight. Ahhh, the wonders of vision. I decided to explore this visual phenomenon.

You’ve seen examples of iridescence—the bright, shimmering, alternating colors of a hummingbird’s throat and feathers, and the reflected swirling rainbow of colors from a soap bubble.

Light interacting with objects in our environment produces sight. The wavelength of the light that is incident on the cone photoreceptors of our retinae determines the color we see. For normal color vision, wavelengths of light reflect from pigments on the surface of objects in our environment, and we see the colors produced by the visible light spectrum; red, orange, yellow, green, blue, indigo and violet. Alternatively, rays reflected from surfaces with multiple refractive layers result in constructive interference of light waves. In constructive interference, the crest and trough of light waves align, which intensifies the reflection of light and the vibrancy of the reflected wavelengths (colors). Constructive interference of light waves amplifies their reflection for a particular wavelength, and when the surface produces iridescence, the light is reflecting varying wavelengths (colors) from varying surface layers and at different angles. Hence the color varies as our angle of view changes, so does the interference both constructive and destructive, explaining why colors alternately appear and disappear. Note: Mirror coating uses the principle of constructive interference to increase reflection, while AR coatings use the principle of destructive interference to eliminate reflection. Fun fact: Iridescence is named after Iris, the Greek goddess of the rainbow who used the rainbow to travel between Mount Olympus and Earth to relay from messages from gods to mortals or other gods.

Deborah Kotob
Pro to Pro Director
[email protected]