A rainbow is a vibrant arc of color that appears when sunlight and rain interact. This phenomenon is a visual representation of what light is composed of, leading to the question of whether the source light’s color—white—is present in the resulting spectrum. Understanding the rainbow requires examining the nature of the light, the mechanics of its formation, and the colors that ultimately appear.
What White Light Really Is
White light, particularly the sunlight that forms a rainbow, is polychromatic, meaning it is a combination of all the different colors of the visible spectrum traveling together. The human eye perceives light within the electromagnetic spectrum ranging from approximately 380 to 750 nanometers. When all these wavelengths are mixed and reach the eye simultaneously, they are perceived as colorless daylight, or white light.
This behavior of light is different from how physical pigments mix, which is subtractive mixing. Light follows the rule of additive mixing, where combining all the spectral colors results in white. White light acts as the complete inventory of colors, containing the full potential for the rainbow.
The Physics of Light Dispersion
The formation of the rainbow results from a three-step process involving sunlight and water droplets suspended in the atmosphere. The process begins when white light enters a spherical raindrop, causing the light to bend, which is called refraction. Light travels at different speeds when moving from air to water, altering its path.
The speed at which light travels through water depends on its wavelength, meaning each color bends at a slightly different angle. This separation of light into its constituent colors is known as dispersion. The light then reflects off the back inner surface of the raindrop and refracts a second time as it exits the droplet, amplifying the separation into the visible arc.
Mapping the Visible Spectrum
The result of the dispersion process is the visible spectrum, the array of colors that compose the rainbow. This spectrum is often remembered by the sequence of red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). Red light, having the longest wavelength, is bent the least, appearing on the outer edge of the arc. Violet light, with the shortest wavelength, is bent the most, forming the inner edge.
The rainbow’s colors are not discrete bands with sharp dividing lines. Instead, the spectrum is continuous, with each color gradually blending into the next. Every wavelength of light between the longest red and the shortest violet is present in the arc.
Why White Light Is Not a Color in the Rainbow
White light is absent from the rainbow’s spectrum because it is the source material from which the rainbow is made. The water droplets act like tiny prisms, separating the white light into its component wavelengths. Once the light is fully separated, the white light ceases to exist as a unified color.
The individual colors of the rainbow represent the full range of the visible spectrum. If an observer could instantaneously collect and recombine all the separated red, orange, yellow, green, blue, indigo, and violet light that makes up the arc, the resulting beam would be perceived as white light. Therefore, white light is not a color within the rainbow, but rather the perfect combination of every color the rainbow displays.