The familiar sight of a rainbow—a single arc of color against a dark sky—is only one manifestation of a complex optical phenomenon. A rainbow is fundamentally the result of sunlight interacting with water droplets, where the light is first bent (refracted), then reflected off the back of the droplet, and separated into its constituent colors (dispersed) as it exits. This basic process, where each tiny water sphere acts as a prism, can produce a variety of arcs, colors, and patterns depending on the number of reflections, droplet size, and the light source.
Fundamental Rainbow Types
The most commonly observed and scientifically understood rainbows result from one or two internal reflections within water droplets.
The Primary Rainbow is the brightest and most distinct arc. It forms when sunlight enters a raindrop, reflects once off the inner surface, and exits toward the observer. This single reflection concentrates the light, causing the bow to appear at an angular radius of approximately 42 degrees from the antisolar point (the spot directly opposite the sun). This process results in the color sequence of red on the outside of the arc and violet on the inside.
The Secondary Rainbow forms when a ray of light is reflected twice inside a water droplet before exiting. This double reflection significantly weakens the light, making the secondary bow appear dimmer. It projects the arc at a wider angle, around 50 to 53 degrees from the antisolar point. The additional reflection also causes the colors to be reversed compared to the primary bow, with red appearing on the inside and violet on the outside.
The space between the primary and secondary rainbows is known as Alexander’s Dark Band. This area appears noticeably darker than the surrounding sky because light cannot be scattered back toward the observer from the raindrops at angles between the two bows.
Variations Caused by Light Interference
Some of the most delicate and scientifically interesting rainbows are explained not by simple reflection but by the wave nature of light.
Supernumerary Bows are a series of faint, repeating arcs that appear just inside the primary rainbow, often showing hues of pink, green, or purple. They are caused by the principles of diffraction and interference, which occur when light waves overlap. This phenomenon is dependent on the size of the water droplets, requiring them to be very small and nearly uniform in size, typically less than one millimeter in diameter. As light rays pass through these small droplets, they follow slightly different path lengths, causing them to constructively and destructively interfere. This interference pattern creates the repeating, pastel-colored fringes adjacent to the main bow.
A Twinned Rainbow is a rare variation where the primary arc appears to split into two distinct bows sharing a common base. Unlike a secondary rainbow, the colors in the second, split arc are in the same order as the primary bow. This occurs because the rain shower contains a mix of two different sizes of raindrops. For example, small, spherical droplets might mix with larger, non-spherical droplets flattened by air resistance. The different exit angles created by the two distinct droplet shapes result in two overlapping, visibly separate, primary arcs.
Rainbows Formed Under Unique Conditions
The appearance of a rainbow can also be dramatically altered when the light source or the medium changes from standard sunlight and rain.
Moonbows, or lunar rainbows, are formed by moonlight reflecting off water droplets. Because moonlight is much fainter than direct sunlight, moonbows are significantly less intense than solar rainbows. Although all colors are present, the faint light is usually insufficient to stimulate the human eye’s cone cells, causing the moonbow to appear ghostly white or gray. They are best seen when the moon is nearly full and low in the sky, positioned opposite a source of moisture like rain, mist, or sea spray.
Fogbows are another type of bow formed by extremely small water droplets found in fog or mist, often less than 0.05 millimeters in diameter. The minute size of these droplets causes the light to diffract heavily, smearing the colors across a broad arc. This heavy smearing results in a wide, diffuse band that appears almost entirely white, earning the common nickname “white rainbow.”
A Reflection Rainbow occurs when sunlight reflects off a smooth, still body of water, such as a lake, before striking falling raindrops. This reflected sunlight acts as a secondary, indirect light source, creating a second complete arc positioned higher in the sky. This extra bow is separate from the normal primary rainbow and intersects it at the horizon. The center of the reflection bow appears above the horizon at the same angle the primary bow’s center is below it.