by Gemma Reguera
The only part of the electromagnetic spectrum that is visible to the human eye is in the narrow region between 390 and 750 nm, which contains all the colors of the rainbow and is referred to as the visible or vis light spectrum (vertical arrow at the bottom). Source.
Our appreciation of the colors of nature is limited by the narrow wavelengths of the electromagnetic spectrum that our eyes can detect. This portion of the electromagnetic spectrum between 390 and 750 nm is what we refer to as the visible (vis) light or, simply, light. We cannot see below (ultraviolet light) or above (infrared light) these wavelengths. Yet this narrow margin of detection allows us to see all the colors of the rainbow, spreading across the visible spectrum from violet (shortest wavelength) to red (longest wavelength). You can see the color palette of visible light in the rainbow up in the sky on a rainy day, as light is dispersed by water droplets. You can also reveal the full color of the visible spectrum using a glass prism: the change of speed of the light as it crosses the glass medium changes the direction of the light waves and enables their dispersion.
Iridescence from soap bubbles (left), shells (middle) and the exoskeleton of a golden stag beetle (right). Source.
Some natural surfaces can produce even more complex optical effects than rain droplets or a glass prism by selectively reflecting light of specific wavelengths and, therefore, specific colors. One optical effect, in particular, the one called iridescence, produces some of the most intense colorations in nature such as the rainbow-like coloration of soap bubbles, the inside of some shells, and the bright colors of the exoskeleton of some insects. The word iridescence originates from the Greek iris, which means ‘rainbow’, and refers to the optical property of some surfaces to change color and its intensity with the illumination or the viewing angle. Iridescent surfaces are uniquely structured in a way that causes the reflected light waves to interact physically with each other. The crests and troughs of the reflected light waves sometimes align (they are ‘in phase’) and reinforce each other, thus increasing the intensity of the reflected color. By contrast, if the reflected light waves are out of phase, they can cancel each other out and those particular colors never manifest. The final effect of this optical interference is the production of one or more predominant colors, the type and intensity changing with the angle of illumination and/or observation. Thus, iridescent colors are ‘structural’: they do not result from pigmentation but from physical interactions between light and surfaces.