The seemingly magical apparition in the sky of different kinds of rainbows has mesmerized many eyes over the millennia. In the Biblical story of Noah’s Ark, the rainbow was a sign from God that He would never again flood the earth destroying its inhabitants. Dorothy Gale in “The Wizard of Oz” sang a beautiful song about what was over the rainbow. Also associated with the Irish tale of there being a pot of gold at the end of the rainbow, these light shows vary in size, shape and color. They may appear by daylight or by moonlight.
Many people have witnessed these amazing natural phenomena not taking the conventional shape or colors of a rainbow, as they are generally known. Why is that? Are these colors in the sky rainbows or are they something else? Science explains that they are indeed rainbows and there is a variety of them for spectators to behold.
How does a rainbow form in the first place? The Physics Classroom tells pupils that the process involves refraction of white light from the sun into a water droplet, internal reflection of the ray of light in the droplet and the refraction of this light out of the water droplet. Since water has a different optical density than air, a light wave will decrease in speed as it enters the water droplet, bending the light wave towards the normal. Upon exiting the water, the light speeds up causing it to then bend away from the normal. Double refraction having occurred, the light ray disperses through the full spectrum of wavelengths. If there is a concentrated volume of mist or water droplets in the atmosphere then someone on the ground, sighting at the right angle, with the sun to their back, will see a colored arc in the sky.
Different types of rainbows form taking into consideration the trajectory of the light ray into the water, the degree of the angle on exit and the observer’s angle of sight. Since the red wavelength is longer than the blue wavelength, people tend to see red at the top of the arc and blue at the bottom. An interesting fact is that no two pairs of eyes see the exact same thing when looking at the same rainbow in the sky.
According to many who have had the opportunity to see a rainbow from an airplane, the bow is actually a circle arc, sometimes a half circle arc. On earth, the ground gets in the way of seeing the full circle. There goes that pot of gold at the end of the rainbow!
Not explicitly observed in the sky, rainbows can form in the spray of an outdoor water hose or sprinkler and in the dew that blankets the grass in the mornings. These last two examples are viewed as a circle when looked at from above.
Here is a list of different kinds of rainbows.
Primary bow: This is what people are most accustomed to seeing in the sky. Rainbow depictions generally follow its example with the colors always appearing in the following order: Red, orange, yellow, green, indigo and blue.
Supernumerary bow: Sometimes when you look inside a primary bow, you will notice fringes of color that are dominant such as greens, purples and pinks. How many fringes appear and the spacing between them vary as we look upon this kind of rainbow.
Secondary bow: This bow is a little different from the others. The rays of light are reflected twice inside the raindrops and two bows form. The second bow’s colors appear inverted; the red band of light is at the bottom and the blue one is at the top.
Alexander's dark band: This is the space between a primary and secondary bow. Raindrops falling between these two bows cannot send any wavelengths to your eyes. What you see is a darker sky within that band.
Red bow: This marvel appearing at sunrise or sunset is quite stunning. The shorter wavelengths such as blue and green are greatly scattered over the long distance they must travel, leaving the longer wavelength of red for your eyes to capture.
Reflection bow: This bow is the product of sunlight being reflected back from a calm surface such as water or wet sand. It has a different bend to it than do other rainbows.
Twinned bow: Amazing to gaze upon, this bow has another smaller bow attached that extends over the top part of the arc giving the appearance of a gap. The reason for this kind of rainbow has not yet been agreed upon.
Sea spray bows: Very exciting to view! Due to the absence of ground, this bow not only reflects in the sky but in seawater spray as well. You do not need a rainbow in the sky to have a seawater rainbow or lake water rainbow. All you need is sunlight at your back and the spray of the water.
Cloud bow: This kind of rainbow is formed by very fine mist in the clouds and the air. Diffraction causes the colors in the rainbow to thin out by broadening the bow. The result is a faint rainbow in the clouds.
Wheel bow: Dense rainstorms or clouds can cast shadows instead of light to our eyes. These shadows seem to radiate out from the horizon, in the center of the rainbow. The effect resembles the spokes on a bicycle wheel and lucky are any who have witnessed this unusual rainbow.
Moon bow: This bow is a truly breathtaking phenomenon. A rainbow seen at night is a very rare occurrence and the colors produced by moonlight are not nearly as bright as those produced by sun rays are.
There are other orders of rainbows, namely 3rd, 4th, 5th, 6th and Zero Order Glow. For more in-depth information about these kinds of rainbows, take a visit to Atmospheric Optics.
A final word. Not all colors in the sky are rainbows. If you happen to see what appears to be a Cheshire cat grinning in the clouds, remember it is not an upside down rainbow and smile back!