Double rainbows are captivating atmospheric displays. They present as two concentric arcs of color. The primary, brighter arc is familiar, while the secondary, fainter arc appears outside it. This phenomenon highlights the intricate interplay between sunlight and water droplets in Earth’s atmosphere.
How Single Rainbows Form
A single, or primary, rainbow forms through a specific interaction between sunlight and countless water droplets. Each tiny droplet acts like a miniature prism. When white sunlight enters a water droplet, it first undergoes refraction, bending and splitting into its individual colors.
After entering the droplet, the light travels to its opposite inner surface. Here, it experiences a single internal reflection. As the light exits the water droplet, it undergoes a second refraction, further separating the colors and directing them towards the observer’s eye. This precise sequence of refraction, reflection, and another refraction creates the vibrant arc of a primary rainbow, with red on the outside and violet on the inside.
The Double Reflection Phenomenon
The appearance of a double rainbow stems from an additional interaction within water droplets. While a primary rainbow results from light reflecting once, a secondary rainbow forms when light undergoes two internal reflections before exiting the droplet. This extra reflection significantly alters the light’s path.
As sunlight enters a water droplet, it refracts and separates into its constituent colors. Instead of reflecting once, this light reflects off the back inner surface, travels across to another inner surface, and reflects a second time. Each reflection causes some light to be lost, and the additional reflection means that less light ultimately exits the droplet to form the secondary bow. This double reflection also causes the light to emerge at a different angle, positioning the secondary arc outside the first.
Why Double Rainbows Look Different
The secondary rainbow exhibits distinct characteristics compared to the primary bow. One noticeable difference is its fainter appearance. More light is scattered or lost with each internal reflection, making the secondary rainbow dimmer than the primary one. Fewer light rays successfully complete the path involving two reflections, contributing to its reduced brightness.
Another characteristic is the reversed order of colors in the secondary rainbow. While the primary rainbow displays colors from red on the outside to violet on the inside, the secondary rainbow shows the opposite sequence, with violet on the outside and red on the inside. This inversion is a direct consequence of the light undergoing an extra reflection within the water droplet. The additional reflection effectively “flips” the color spectrum.
Spotting a Double Rainbow
Observing a double rainbow requires specific atmospheric conditions and an optimal viewing position. The sun must be low in the sky, typically in the early morning or late afternoon, and positioned behind the observer. This low sun angle allows the light to interact with raindrops at the necessary angles for both primary and secondary bows to form.
Rain must be present in the opposite direction from the sun, creating the water droplets needed to disperse and reflect sunlight. Clear skies in the direction of the sun are also helpful, ensuring that sunlight can reach the raindrops unobstructed. While primary rainbows are relatively common, the conditions for a clearly visible secondary rainbow are less frequent, making their appearance a more special event.