A rainbow is a meteorological event that appears when sunlight interacts with water droplets suspended in the atmosphere. This display results from a combination of light reflection, refraction, and dispersion. The phenomenon transforms white sunlight into a spectrum of distinct hues arranged in a fixed pattern.
The Universal Sequence of Colors
The sequence of colors observed in every primary rainbow is consistent across the globe. This arrangement begins with red on the outermost edge of the arc, followed by orange, yellow, green, blue, indigo, and finally violet on the innermost edge. This specific set of seven colors represents the visible spectrum of light.
The colors are not discrete bands but a continuous transition of hues, blending smoothly from one to the next without clear boundaries. The consistent appearance of this specific sequence confirms that the process creating the rainbow is governed by fixed physical laws. The order is immutable, with red always appearing at the largest radius and violet always appearing at the smallest radius of the arc visible to the observer.
The Physics Behind the Order
The specific arrangement of the rainbow’s colors is a direct consequence of how light behaves when it passes through water droplets. When white sunlight enters a raindrop, it slows down and bends, a process known as refraction. The light then reflects off the back inner surface of the droplet and refracts again as it exits, sending the light back toward the observer. The angle at which the light emerges determines which color the observer sees.
The factor determining the color order is dispersion, which occurs because the speed of light varies depending on its wavelength. Shorter wavelengths, such as violet and blue, slow down more significantly when entering the water droplet. This greater change in speed causes them to refract at a larger angle compared to longer wavelengths. This difference in bending angles separates the spectrum.
Conversely, longer wavelengths, like red light, experience less change in speed and bend at the smallest angles. Violet light emerges at the lowest angle—approximately 40 degrees—relative to the incoming sunlight, placing it on the inner boundary of the rainbow arc. Red light, having bent the least, emerges at the highest angle—approximately 42 degrees—positioning it along the rainbow’s outer edge.
This differential bending means that each color is directed toward the observer at a slightly different angular range, separating the white light into its constituent components. The result is the distinct spectral band where red is consistently outside and violet is consistently inside.
Remembering the Sequence
To quickly recall the fixed sequence of colors, a handy memory tool, or mnemonic, has been widely adopted by students and scientists. This tool uses the first letter of each color to form the name “ROY G. BIV.” The letters stand for Red, Orange, Yellow, Green, Blue, Indigo, and Violet, in their proper order from the outer to the inner edge of the primary rainbow. The use of this simple mnemonic ensures the correct color arrangement can be recalled effortlessly.