Rainbows have long captured human imagination. These atmospheric displays emerge under specific natural conditions. Understanding how and when they form can increase the likelihood of witnessing this phenomenon.
The Physics of Rainbow Formation
Rainbows begin with the interaction of sunlight and water droplets. When white sunlight enters a spherical raindrop, it slows down and changes direction (refraction). This initial refraction splits the white light into its constituent colors, much like a prism, because each color bends at a slightly different angle.
The separated light travels to the raindrop’s back inner surface, undergoing internal reflection. After reflection, the light travels to the front of the droplet and exits, undergoing a second refraction. This second bending further disperses the colors and directs them back towards an observer, creating the visible spectrum of a rainbow.
Ideal Time and Conditions for Viewing
Observing a primary rainbow depends on the alignment of the sun, water droplets, and the observer’s position. The sun must be behind the observer and low in the sky, less than 42 degrees above the horizon. This means the most favorable times for viewing are in the early morning or late afternoon. The lower the sun is, the more of the rainbow’s arc will be visible above the horizon.
For a rainbow to appear, there must be rain or mist opposite the sun. The observer needs to be positioned between the sun and the rain. Each individual sees their own unique rainbow, formed by light from specific raindrops at the correct angle relative to their eyes.
A rainbow appears at an angle of 40 to 42 degrees from the antisolar point, the point directly opposite the sun. This angular relationship is why rainbows always appear as arcs, with the center of the arc directly aligned with the observer’s shadow. A clear sky opposite the sun helps to enhance the visibility of the rainbow’s colors.
Uncommon Rainbow Sightings
Beyond the familiar primary rainbow, several less common optical phenomena exist. Double rainbows form when sunlight undergoes two internal reflections within the raindrops. The secondary bow appears outside the primary one, is fainter, and exhibits a reversed color order, with red on the inside and violet on the outside. A dark band, known as Alexander’s band, separates the primary and secondary rainbows.
Fogbows resemble rainbows but form in fog or mist rather than rain. The water droplets in fog are smaller than raindrops, which causes light to diffract extensively. This diffraction smears out the colors, making fogbows appear predominantly white or faint hues. Like rainbows, they require the sun to be behind the observer and the fog bank in front.
Rainbows can also be observed from sources of fine water mist, such as garden sprinklers or waterfalls. These localized rainbows occur when sunlight interacts with the spray, demonstrating the same principles of light refraction and reflection. The observer needs to have the sun at their back to see these displays.
Moonbows, also known as lunar rainbows, are a rare nocturnal phenomenon created by moonlight interacting with water droplets. Because moonlight is dimmer than sunlight, moonbows are fainter and appear white or gray, as the light is too dim to stimulate color receptors. Ideal conditions for moonbows include a bright, nearly full moon low in the sky (less than 42 degrees above the horizon), a dark night, and water droplets opposite the moon.