7 types of rainbows: Nature's mesmerizing optical phenomena

• A rainbow is a natural optical phenomenon that occurs when a source of light strikes water droplets.
• Rainbows are actually full circles. However, from the ground, they generally appear as arcs.
• There are several types of rainbows – not just the multicolored bright arc frequently observed when the sun comes out on a rainy day.

Rainbows are one of nature's most mesmerizing and awe-inspiring phenomena. Whether you're an engineer, scientist, artist, or simply someone who appreciates the beauty of nature, you most likely agree there is something captivating about rainbows that make them somewhat impossible to ignore. 

But did you know that there is more than one type of rainbow? Most people don't. In fact, not all rainbows resemble the bright multicolored arc we frequently see when the sun comes out on a rainy day. This article looks at all the different varieties of rainbows and the conditions that must be met for each one to exist.

What is a rainbow?

A rainbow is a natural optical phenomenon that occurs when light is refracted or bent as it passes through airborne water droplets. The light is broken up into its component wavelengths, creating a spectrum of colors that appears to form a circular arc in the sky.

Rainbows can be caused by various light sources, including sunlight, moonlight, and even streetlights, but the most common source is sunlight.

How are rainbows formed?

Firstly, let's understand two things; refraction gives you the colors of the rainbow, and reflection gives you its round shape.

When light is refracted or bent at an angle, as it passes through water droplets in the air, it is separated into its component wavelengths, as each is reflected at a different angle, creating a spectrum of colors. The amount the light bends depends on its wavelength. So, shorter wavelengths, such as blue and violet, are bent at a greater angle than the longer wavelengths, such as red. Thus, the spectrum is separated, producing a rainbow.

Why are rainbows curved?

Why a rainbow is curved relates to the second concept highlighted above; reflection. When sunlight hits an airborne water droplet, some of that light bounces back or is “reflected.”

As a result, when you see a rainbow, what you're truly seeing is light that has struck a droplet and returned to your eye. Normally, this is done at an angle of 40 to 42 degrees.

Say we draw sunlight rays reflecting at 42 degrees back to your eyes. In that case, those rays start to look like they form a circular arc in the sky.

Still, all rainbows are actually full circles, with the antisolar point (the imaginary point exactly opposite the Sun) at the center of the circle. If you are standing on the ground, you can only see the light reflected by raindrops above the horizon. But on the off chance that you're on a plane or high on a mountain and a rainbow appears, you might be able to view it as a whole circle – not just a portion of it.

The rainbow's radius depends on how much the light refracts, or bends, as it passes from one medium to another (such as air to water). The higher the refraction, the smaller the radius of the rainbow.

What does all of this mean, then? Well, sorry to break the bad news, but there isn't treasure at the 'end' of the rainbow. After all, a rainbow is a whole circle; there are no ends.

What are the colors of a rainbow?

Most rainbows are composed of the seven colors of the visible spectrum: red, orange, yellow, green, blue, indigo, and violet. In a primary rainbow (discussed below), because the wavelength of the colors doesn't change, they always appear arranged in a specific order, with red on the outer edge and violet on the inner edge. To remember them, they are sometimes referred to as "ROYGBIV." 

The exact colors visible in a rainbow can vary depending on the angle of the light and the size of the water droplets, as well as the location of the person viewing them. 

What are the different types of rainbows?

Rainbows come in various forms, each with its own unique qualities. Here, we will go over seven different types, which is by no means an exhaustive list.

1. A primary 'solar' rainbow

A primary rainbow is the most common type of rainbow produced by the sun (or solar light). It occurs when light is refracted or bent as it passes through water droplets in the atmosphere. This bending causes the light to be separated into its various wavelengths (colors), which are then reflected back to the observer in a circular arc. 

One of the most striking features of a primary rainbow is that it always appears in the opposite part of the sky from the Sun. The light must be refracted at a specific angle for the colors to be separated and reflected back to the observer. 

2. Double rainbows

A double rainbow is when two separate, concentric rainbows appear parallel to one another. It is actually a relatively common sight, although no less spectacular, and is most common when the sun is low in the sky, such as in the early morning or late afternoon.

In a double rainbow, the light is reflected twice at slightly different angles when there are different sizes and shapes of water droplets present in the atmosphere. The double reflection produces two rainbows. 

One of the most striking features of a double rainbow is the presence of a second band of colors, sometimes referred to as the secondary rainbow, which is typically higher, wider, and fainter than the primary rainbow. This is because more light escapes from two reflections compared to one.

Look closely, and you'll find that the colors of the secondary rainbow are arranged in the opposite order of the primary rainbow. In other words, violet is on the outermost band, and red is on the innermost (VIBGYOR).

3. A twinned rainbow

In extremely rare circumstances – two rainbows (not running parallel to each other) may be visible arching through the sky. Unlike double rainbows, rainbow arcs appear to split from a single base. In addition, the colors in the second rainbow appear in the same order as those in the primary rainbow rather than in reverse order.

Alexander's band is the name given to the unlit area that lies between two rainbows (the name applies to double rainbows, too), which appears subtly darker than the rest of the sky. It's a name derived from Alexander of Aphrodisias, a philosopher from the second century who was the first to describe this phenomenon.

4. A moonbow

A moonbow, also known as a lunar rainbow, is a rarer type of rainbow created by moonlight rather than sunlight. It occurs when light from the Moon is refracted or bent as it passes through water droplets in the atmosphere. These water droplets typically come from a rain shower or storm during the night. 

Similar to a rainbow created by sunlight, a moonbow is a circular band of colors created when the light is separated into various colors. 

One of the most striking features of a moonbow is its faintness, as the Moon does not produce as much light as the Sun. Therefore moonbows are typically much dimmer than solar rainbows. Moonbows are also harder to see than rainbows because the human eye is less sensitive to the colors of the spectrum in low light conditions.

5. A fogbow

The presence of rain is not even necessary for a rainbow to appear – at least not for a fogbow. This is formed when sunlight passes through the water droplets that make up mist and fog. The light is spread out much more than in a solar rainbow, and, like a moonbow, a fogbow's circular shape consists of very faint colors. 

6. A red rainbow

The only type of rainbow covered here that is not multicolored is a monochrome or red rainbow. The same phenomenon causes its formation as that of a solar rainbow; water droplets reflect or refract light. The difference is that the Sun must be low in the sky for a monochrome rainbow to occur. Usually, this happens at sunrise or sunset. 

The Sun's light must travel a longer distance through the atmosphere due to its low angle. This scatters the shorter wavelengths of light, such as blue, green, and yellow, leaving primarily red.

7. An upside-down rainbow

Some rainbows even appear upside down. For instance, a circumzenithal arc, or upside-down rainbow, is created when sunlight passes through ice crystals in high-level cirrus clouds. The precise angle at which the light strikes the ice and the angle at which the observer views it produces this effect.  

Rainbows: concluding notes

In conclusion, rainbows are one of nature's most fascinating and awe-inspiring phenomena. They are created by the process of refraction, or the bending of light as it passes through water droplets in the atmosphere. The bending of light causes the light to be separated into its various colors and reflected back to the observer in a circular arc (depending on the viewpoint).

Rainbows come in many types, and not all need 'rain.' Each one is unique and spectacular in its own way, offering a glimpse into the complex and fascinating ways light interacts with the world around us.