A rainbow is a natural optical phenomenon that appears when sunlight interacts with moisture in the atmosphere. It forms a multicolored arc in the sky, always appearing opposite the sun. Understanding this requires looking closely at the physics of light and how it behaves when it encounters water. The spectacle requires two main ingredients: a source of white light, typically the sun, and numerous airborne water droplets.
White Light is a Spectrum
The light we receive from the sun appears white, but it is actually a composite of all the colors that make up the visible electromagnetic spectrum. Light travels in waves, and each distinct color corresponds to a specific wavelength. The visible spectrum spans from approximately 400 nanometers (violet) up to around 750 nanometers (red).
This range includes the continuum of colors seen in a rainbow, with red having the longest wavelength and violet having the shortest. When all these different wavelengths are combined, as they are in sunlight, the result is the perception of white light.
How Water Droplets Act as Prisms
Tiny water droplets in the air, such as rain or mist, separate the sun’s white light into its component colors. Each spherical droplet acts like a miniature prism. When sunlight strikes the surface of a droplet, it slows down as it passes from air into water, causing it to bend.
This bending of light as it changes medium is known as refraction. As the light travels through the water, it hits the back interior surface of the droplet, where it undergoes a single reflection. The light is then refracted a second time as it exits the water and re-enters the air toward the eye.
The degree to which light bends upon refraction depends on its wavelength, a process called dispersion. Since the bending angle is slightly different for each color, the light rays spread out upon entering and exiting the droplet. This separation of wavelengths converts the initial white light into a spectrum.
Why Colors Follow a Specific Sequence
The consistent, ordered sequence of colors—red, orange, yellow, green, blue, indigo, and violet—is a direct result of how wavelength affects the light’s path through the water droplet. Light with a longer wavelength, such as red light, bends the least when refracted. Conversely, light with a shorter wavelength, like violet light, bends the most.
This difference in bending angle means each color emerges from the water droplet at a slightly different angle relative to the incoming sunlight. Red light exits the droplet at the largest angle, around 42 degrees, while violet light exits at a smaller angle, approximately 40 degrees. This specific angular separation determines the color sequence that reaches the observer’s eye.
To see a specific color, the observer must be looking at water droplets positioned at the exact angle corresponding to that color’s emergence. The red light from droplets on the outer edge of the arc reaches the eye, while the violet light comes from droplets on the inner edge.