Triple rainbows are an atmospheric phenomenon, theoretically possible but exceedingly rare to witness. Their existence is confirmed by the physics of light interacting with water droplets, though observing them presents significant challenges due to their extreme rarity.
How Rainbows Form
A rainbow appears when sunlight interacts with water droplets suspended in the air. This process involves three main optical principles: refraction, reflection, and dispersion. As white sunlight enters a spherical raindrop, it bends, or refracts, because light travels slower in water than in air. This refraction causes the light to separate into its constituent colors, a process known as dispersion.
After entering the droplet, the light travels to the back inner surface of the raindrop. Here, it undergoes a single internal reflection. Finally, the light refracts a second time as it exits the water droplet and re-enters the air, further separating the colors and directing them towards an observer. This sequence creates the familiar primary rainbow, with red on the outermost edge and violet on the innermost edge.
The Double Rainbow Phenomenon
Double rainbows occur when sunlight undergoes a slightly different path within the water droplet. Instead of a single internal reflection, the light reflects twice inside the raindrop before exiting. This additional reflection causes the light to emerge from the droplet at a different angle compared to the primary rainbow, typically appearing about 10 degrees above it.
The secondary rainbow is noticeably fainter and more pastel in tone than the primary bow. This reduced intensity is due to more light escaping with each additional reflection within the droplet. A distinguishing characteristic of a double rainbow is the reversed order of colors in the secondary arc, with violet on the outside and red on the inside. Between the primary and secondary rainbows, a dark band often appears, known as Alexander’s Dark Band, where no light is scattered towards the observer.
The Elusive Triple Rainbow
The theoretical formation of a triple rainbow, also known as a tertiary rainbow, requires sunlight to undergo three internal reflections within water droplets. This multi-reflection process makes them faint, as each additional internal reflection causes a significant loss of light.
A primary reason for their extreme rarity is the viewing angle required. Unlike primary and secondary rainbows that appear opposite the sun, a true tertiary rainbow forms around the sun itself. This means an observer would need to look directly towards the sun to spot it, making it difficult to discern against the bright glare. Specific atmospheric conditions are also necessary, including a very bright sun and a heavy downpour of uniformly sized raindrops. Before 2011, only five scientific reports of triple rainbows existed over 250 years, highlighting their exceptional nature.
Identifying Rare Rainbows and Related Phenomena
Observing a genuine triple rainbow is extraordinarily difficult due to its extreme faintness and the requirement to look towards the sun’s glare. The specific atmospheric conditions needed, such as large, uniform raindrops and a dark background of thunderclouds, further limit opportunities for sightings. Confirmed photographic evidence of true tertiary rainbows only emerged in 2011, following meteorological predictions on where and how to find them.
Other optical phenomena can sometimes be mistaken for multiple rainbows. Supernumerary rainbows appear as faint, extra bands of color just inside the primary rainbow, caused by the wave nature of light and interference patterns. Reflection rainbows, on the other hand, form when sunlight reflects off a large, calm body of water before hitting raindrops, creating an additional arc that intersects the main rainbow at the horizon. These distinct phenomena differ in their formation from true higher-order rainbows.