Rainbows captivate observers with their vibrant arcs across the sky. This optical phenomenon has long inspired curiosity about the precise mechanisms that create such a vivid display of colors. Understanding the science behind rainbows reveals how ordinary sunlight transforms into a breathtaking spectrum.
The Science Behind Rainbows
Rainbows form when sunlight interacts with water droplets suspended in the atmosphere. Each tiny water droplet acts like a miniature prism, bending and separating white light into its constituent colors. This process begins with refraction: as sunlight enters a water droplet, it slows down and changes direction.
After entering the droplet, the light reflects internally off its back surface. It then refracts again as it exits and travels towards an observer’s eyes. This double refraction and single reflection within numerous water droplets cause the separation of white light into a continuous band of colors. Different wavelengths of light bend at slightly different angles, leading to their dispersion.
Understanding Spectral Colors
The colors observed in a rainbow are known as spectral colors, produced by single wavelengths of light within the visible spectrum. These colors emerge when white light is split. The commonly recognized spectral colors are red, orange, yellow, green, blue, indigo, and violet. Each corresponds to a specific range of electromagnetic wavelengths.
Red light has the longest wavelength (625-750 nanometers), while violet light has the shortest (380-450 nanometers). These spectral colors are continuous, with no distinct boundaries between them within the rainbow’s arc. They represent the fundamental components of visible light that our eyes can detect.
Why Pink Is Different
Pink is not a spectral color and does not appear in a rainbow. Spectral colors are generated by a single, distinct wavelength of light, but pink does not have its own unique wavelength. Instead, pink is a perceptual color, created by our brains when specific light combinations stimulate our eyes. It is essentially a mixture, typically involving red light and blue or violet light, often with a desaturated quality.
When the human eye receives strong signals from red-sensitive and blue-sensitive cones, but not much green, the brain interprets this combination as pink. This means pink is an “extra-spectral” color, formed by combining wavelengths from opposite ends of the visible spectrum. Unlike spectral colors, pink is a construct of our visual system, filling a perceptual gap between red and violet.