Rainbows are vibrant arcs of light that inspire wonder. While a single rainbow is beautiful, a second, fainter arc, known as a double rainbow, is even more striking. This display often prompts curiosity about its formation and why it’s observed less often than its solitary counterpart.
The Science of Double Rainbows
A rainbow forms when sunlight interacts with water droplets. Sunlight enters a raindrop, refracts, and splits into colors. It then reflects off the inner back surface and refracts again as it exits. This single internal reflection creates the primary rainbow, displaying colors from red on the outside to violet on the inside.
A double rainbow, or secondary rainbow, forms when sunlight reflects twice inside a raindrop. This double reflection reverses the color order compared to the primary rainbow, showing violet on the outside and red on the inside. The secondary rainbow forms at a larger angle, positioned about 9 to 10 degrees outside the primary one.
Why Double Rainbows Are Less Common
Double rainbows appear less frequently due to light loss during the second internal reflection. Each reflection causes light to escape or scatter, making the secondary rainbow significantly fainter and more pastel than the primary one. This faintness makes it harder to discern against the sky.
Specific atmospheric and observational conditions are also necessary for clear visibility. The sun must be relatively low in the sky, typically at an angle of less than 42 degrees above the horizon, often during morning or late afternoon. Sufficient rain must be falling opposite the sun. Additionally, the sky behind the observer needs to be clear for an unobstructed view.
The space between the primary and secondary rainbows often appears darker, a phenomenon known as Alexander’s band. This dark band occurs because sunlight is not scattered from raindrops at angles between the two bows. Though a secondary rainbow is theoretically always present, its faintness and specific conditions mean it is not always noticeable.
How to Spot a Double Rainbow
To increase your chances of observing a double rainbow, timing and positioning are important. Look for them after a rain shower when the sun is low in the sky, such as in the early morning or late afternoon.
Stand with your back to the sun, facing the rain or mist. This alignment ensures light reflects from raindrops directly towards your eyes. Clear skies opposite the sun provide a better backdrop, making the fainter secondary rainbow more discernible. Clear observation requires favorable light, rain, and viewing conditions.