Environment

How Is a Rainbow Formed?

Refraction, reflection, and dispersion of light in water droplets result in the formation of rainbows.

What Is a Rainbow?

A rainbow is a multi-colored, arc-shaped phenomenon that can appear in the sky. The colors of a rainbow are produced by the reflection and dispersion of light through water droplets present in the atmosphere. An observer may perceive a rainbow to be located either near or far away, however, this phenomenon is not actually located at any specific spot. Instead, the appearance of a rainbow depends entirely upon the position of the observer in relation to the direction of light. In essence, a rainbow is an optical illusion.

Rainbows present a spectrum made up of seven colors in a specific order. In fact, school children in many English-speaking countries are taught to remember the name “Roy G. Biv” as a mnemonic device for remembering the colors of a rainbow and their order. “Roy G. Biv” stands for: red, orange, yellow, green, blue, indigo, and violet. The outer edge of the rainbow arc is red, while the inner edge is violet.

How Is a Rainbow Formed?

A rainbow is formed when light (generally sunlight) passes through water droplets hanging in the atmosphere. The light waves change direction as they pass through the water droplets, resulting in two processes: reflection and refraction. When light reflects off a water droplet, it simply bounces back in the opposite direction from where it originated. When light refracts, it takes a different direction. Some individuals refer to refracted light as “bent light waves.” A rainbow is formed because white light enters the water droplet, where it bends in several different directions. When these bent light waves reach the other side of the water droplet, they reflect back out of the droplet instead of completely traversing the water. Since the white light is separated inside of the water, the refracted light appears as separate colors to the human eye.

Colors of the Rainbow

Each individual wave of color has a different length. For example, red light has the longest wavelength and only bends at about a 42-degree angle. Violet light, in contrast, has the shortest wavelength and bends at around 40 degrees before exiting the water droplet. Because the red light wavelength is longer, it most commonly appears on the outside edge of the rainbow. Similarly, the other colors are also ordered according to their wavelength. Other waves of light are also reflected from the rainbow, however, these light waves are not visible to the naked human eye. These invisible rays are present on both sides of the rainbow. Ultraviolet rays are shorter than violet rays and x-rays are even shorter than ultraviolet rays. Gamma radiation is at the furthest extreme of this side of the rainbow. At the other end of the spectrum is infrared radiation and radio waves.

Types of Rainbows

Rainbows are formed in a number of ways. Some of the various types of rainbows are highlighted below:

Double Rainbow

A double rainbow occurs when a second rainbow is visible above the principal rainbow. The second rainbow is not as bright as the first. This phenomenon is made possible by double reflection, which causes the color order of the second rainbow to be reversed.

Moonbow

Although most rainbows are associated with sunlight occurring immediately after a rain shower, some rainbows are created by the light of the moon. Moonbows are less common than daylight rainbows. These illusions can only be seen in some areas of the world, typically where waterfalls are located. Moonbows are often seen in the spray created toward the bottom of these falls. Additionally, moonbows usually require the light of the full moon to be visible. Most people view moonbows as completely white.

Fogbow

Like moonbows that typically occur in waterfall spray, fogbows can be seen in instances of thin fog combined with significant sunlight. In this case, light reflects off a dense collection of water particles, which results in a wide and bright rainbow. Fogbows are almost entirely white in color. This white appearance occurs because each light wave is projected over a very wide area. These wide streaks tend to blend together, creating the white color. However, red and blue streaks of color can sometimes be seen along a fogbow's edges.

Reflection Rainbow

Reflection rainbows can be seen above large bodies of still water, such as lakes. These reflections occur when a primary rainbow is visible over the surface of water. The water reflects the primary rainbow, creating a secondary rainbow above the primary. This secondary rainbow is only a reflection of color and is somewhat fainter than the primary rainbow. Its shape takes on an elongated form and usually stretches upwards in a straight line, rather than in an arc shape. These two rainbows appear to touch where each meets the earth, creating a wider and brighter section of the phenomenon. Reflection rainbows are uncommon.

Reflected Rainbow

A reflected rainbow is similar to a reflection rainbow in that it occurs over a large body of still water, although some individuals have reported observing reflected rainbows in smaller collections of still water as well. The difference between these two types of rainbows is that the reflection is not projected into the sky, but rather over the surface of the water. These rainbows are formed when waves of light pass through water droplets in the atmosphere and are reflected in the surface of the water. The end points of both the primary and reflected rainbow appear to touch in the water, however, the two do not form a complete circle. Instead, the reflected rainbow creates an elongated oval-type shape with the rainbow in the sky.

Monochrome Rainbow

As its name suggests, a monochrome rainbow takes on one solid color rather than the full spectrum typically observed in rainbows. This phenomenon produces a solid red rainbow. These rainbows are more common after a rainfall that occurs close to sunset or sunrise. At these hours, sunlight travels deeper into the atmosphere, causing green and blue light waves to be spread over a wider area, and without these colors red light waves are able to dominate the sky. Monochrome rainbows are considered a rare phenomenon.

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