A rainbow is a spectacular optical phenomenon appearing as a multicolored arc in the sky. It is not a physical object but a visual effect dependent on the observer’s position relative to the sun and atmospheric moisture. The arc forms when sunlight interacts with water droplets suspended in the air. When the sun is positioned behind the observer and mist is present in front, the conditions are right for light to be scattered. This interaction separates the white light into its constituent colors.
The Seven Primary Colors of Light
The visible colors that compose a rainbow have been traditionally grouped into seven distinct hues. These colors, in their consistent order from the outside to the inside of the arc, are Red, Orange, Yellow, Green, Blue, Indigo, and Violet. This specific sequence is a direct result of how light energy is sorted by water droplets in a primary rainbow. Red always forms the outermost band of the arc, while violet is seen on the innermost edge.
To easily recall this sequence, the popular mnemonic device “Roy G. Biv” is used. Each letter stands for the first letter of the seven colors in the correct order. Although the visual spectrum contains far more colors than seven, this grouping has remained the standard description since the 17th century.
How Water Droplets Separate Light
The separation of white sunlight into its various colors is achieved through a three-step process involving water droplets acting as tiny prisms. The process begins with refraction, the bending of light as it passes from air into the denser water droplet. As the light enters, it slows down and changes direction.
This initial bending is accompanied by dispersion, where different wavelengths of light bend by slightly different amounts. Red light, which has the longest wavelength, is bent the least, while violet light, with the shortest wavelength, is bent the most. This difference in bending angles sorts the white light into a spectrum.
After entering the droplet, the light travels to the back surface, where it undergoes internal reflection, bouncing off the inner wall. The light then travels back toward the front, experiencing a second instance of refraction as it exits back into the air. This second bending further enhances the separation of colors.
The combined effect of the two refractions and one internal reflection causes the colors to emerge at specific angles relative to the incoming sunlight. Red light emerges at approximately 42 degrees, and violet light at about 40 degrees. Because red light is directed toward the observer at a higher angle than violet, red is perceived on the outer edge of the arc, and violet is seen on the inner edge.
The Continuous Nature of the Spectrum
While the rainbow is traditionally described by the seven colors of “Roy G. Biv,” the visible light spectrum is a smooth, uninterrupted gradient. The light moves continuously from the longest visible wavelength (red) to the shortest (violet). Every hue gradually blends into its neighbor, creating countless shades the human eye cannot fully distinguish.
The specific division into seven colors was popularized by Sir Isaac Newton in the 17th century. He initially identified five colors but later added orange and indigo to reach seven. This choice was based on the belief that the number of colors should connect to the seven notes of the musical scale, making it more historical than strictly scientific.
The inclusion of “indigo” is particularly debated, as it is difficult to isolate visually between blue and violet, and many modern scientific models often omit it. The colors we see are merely convenient markers along the smooth progression of light wavelengths.