Rainbows are a natural display of light and water, prompting questions about their colors. The absence of certain colors, like pink, often leads to exploring the scientific principles governing how we see light.
The Colors of a Rainbow
Rainbows form when sunlight interacts with water droplets in the atmosphere, a process known as dispersion. As white sunlight enters a raindrop, it slows and bends, or refracts, upon entry. The light then reflects off the back inner surface of the raindrop and refracts again as it exits. Each color of the visible light spectrum, from red to violet, has a slightly different wavelength, causing them to bend at unique angles. This difference in bending angles separates the white light into its component colors, much like a prism.
Rainbows consistently display colors in a specific order: red, orange, yellow, green, blue, indigo, and violet. These seven colors represent the visible spectrum of light. Red light has the longest wavelength and bends the least, appearing at the top or outer edge of the rainbow arc. Violet light, with the shortest wavelength, bends the most and is positioned at the bottom or inner edge. This continuous spread of spectral colors showcases sunlight’s inherent composition.
Why Pink Isn’t a Rainbow Color
Pink, or magenta, is notably absent from a rainbow’s distinct color bands because it is not a spectral color. Spectral colors exist as a single, pure wavelength of light, like red or blue. A rainbow displays the visible spectrum by breaking light into these individual wavelengths. Since pink lacks a unique wavelength, it cannot be separated and displayed.
Instead, pink is an extra-spectral color, a perception created by our brains when specific light combinations stimulate our eyes. For pink to be perceived, a mixture of red light and violet or blue light must be present. These two ends of the visible spectrum are typically far apart in a rainbow’s arc, and their wavelengths do not combine to produce pink. Our visual system processes these non-adjacent spectral inputs to construct the perception of pink.
Rainbow colors result from light separating into its constituent wavelengths. As pink is a composite color, an interpretation by the brain rather than a distinct wavelength, it does not naturally appear when white light disperses. Thus, while our eyes perceive pink in other contexts, rainbow formation ensures only spectral colors are displayed.