Different myths explain the existence and formation of a rainbow. However, modern meteorologists like Author Donald Ahrens describe the rainbow as a spectacular light that occurs when sunlight passes through water droplets in the atmosphere.
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While the rainbow occurs when light passes though the atmospheric moisture, the arc shaped spectrum of the rainbow results from the refraction of sunlight. Apparently, as sunlight passes through the water drops, it does not only produce the rainbow, but also produce a variety of visual spectacles such as moon bows, coronas, fogbows, and the sweet-colored sunrises and sunsets.
Observation of the rainbow (Department of Atmospheric Sciences, 2011, p. 531)
In essence, a rainbow forms the sun is shining from one part of the sky while rainfalls, mist, or air borne dew are on the other side of the sky (Naylor, Lynch, & Livingston, 2002, p. 23). A person can only see a rainbow at a 40o angle of incidence towards the suspended water droplets or mist while facing the sun. From the optician explanation, the rainbow is non-tangible and non-existing phenomenon.
The rainbow is simply a distorted image of the sunlight caused by refraction, reflection, and refraction of sunlight. The brilliant colors of the rainbow follow the order of red, orange, yellow, green, blue, indigo, and violet. Thus, this report examines the rainbow in terms of its formation, explanation of its different colors, and the visual spectacles whose formation resembles that of the rainbow.
The Different Colors of the Rainbow
The different colors of the rainbow occur due to the difference in the wavelengths of sunlight. The raindrops in the atmosphere acts as a prism or rather an imperfect mirror that reflects, refracts, and disperses light. The white ray from the sun hits the raindrop, and its refraction causes it to split into different colors that form a rainbow. The different colors result from the optical color scattering, which depends on the wavelength and the refractive index.
Refraction and dispersion of light (Department of Atmospheric Sciences, 2011, p. 528)
The violet color, for example, results from a wavelength of 400nm and a refractive index of 1.3445. A wavelength of 525nm with a refractive index of 1.33659 gives a green color. The trend continues with an increasing wavelength, where, a 700nm wavelength with a refractive index of 1.33141 gives a red color.
The red color has an angle of 42° 2′, while the violet has an angle of 40° 17′ (Minnaert, Lynch, & Livingston, 2006, p. 19). If the sunlight is parallel, the difference between the angles of the red and violet rainbow will give the size of the rainbow as 1° 45′. However, this is not the case as the rainbow is 2° 15′ because of the half-degree diameter of the sun disk.
Formation of the Rainbows: Sunlight Refraction, Reflection, and Dispersion
The three phenomena of sunlight refraction, sunlight reflection, and dispersion offer an explanation behind the formation of a rainbow. As a rainbow demonstrates excellent dispersion of light as a spectrum of wavelengths, the following steps summarizes its formation:
- Light rays from the sun: The rainbow occurs mostly when the sun is low, that is late in the afternoon, or early in the morning. White rays of sunlight meet raindrops at an angle of incidence that causes refraction, reflection, and dispersion of light.
Scattering of light (Department of Atmospheric Sciences, 2011, p. 519)
Partial reflection of light: The water droplets transmit and reflect part of the light, but refract the rest of the light (Greenler, 2000, p. 3). Refraction happens because the speed of light slows down as it bends towards the normal line.
- Light splitting: The sun’s white light contains a spectrum of colors with wavelengths of different speeds. Upon encounter with the prism (water in this case), the wavelength’s speed changes depending on the density of the water, thus causing dispersion (Graham, 2007, p. 45). Dispersion is a phenomenon of separating colors caused by the change in the speed of the wavelengths.
- Total internal reflection and refraction: When the angle of incidence exceeds the critical angle, total internal reflection and refraction happens as light passes through a raindrop. The refraction occurs because sunlight moves from a high-density medium (raindrop) to a low-density medium (air). This causes yet another refraction, which results into further dispersion of colors making the rainbow to appear clear.
The Types of Rainbows
The Primary Rainbow
The primary and secondary rainbows (Department of Atmospheric Sciences, 2011, p. 532).
It forms at about 40° to 42° from the anti-solar point. As described before, refraction, reflection, and dispersion are the processes behind the formation of the rainbow. The primary rainbow occurs when water droplets cause a single reflection of sunlight and a subsequent single refraction.
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In the case of large droplets that exceed 1-millimeter, red color, green, and violet colors associated with some little blue form. Mist causes the disappearance of all colors except the violet color, while small raindrops weaken the formation of a red color. Whenever there are fine fog droplets, a white rainbow forms.
Violet, indigo, blue, green, yellow orange and red are the sequence of colors that are in the primary rainbow from the inner to outer bow. The violet color is visible at 40o while the red color is visible at 42o, thus explaining why the rainbow is not visible at midday as the 42o circle is usually below the horizon at midday.
The Secondary Rainbow
The secondary rainbow occurs above the primary rainbow at about 50° to 52° from the anti-solar point. The secondary rainbow has colors with lower intensity than that of the primary rainbow. The intensity of the secondary rainbow is approximately 10% that of the primary rainbow. The secondary rainbow occurs when light enters a raindrop and undergoes a double reflection (Graham, 2007, p. 26). This explains the occurrence of the fainted colors in the secondary rainbow.
Moreover, the width of the secondary rainbow is twice as much as that of the primary bow, but it has reversed colors. Unlike the primary rainbow, the secondary rainbow obtains its violet color from heavy raindrops, while the red color comes from light drops. In the formation of secondary rainbow, the sequence of colors is that violet color is at the outer arc, while indigo, blue, green, yellow, orange, and red follow towards the inner arc. A somewhat dark region separates the primary and the secondary rainbows.
The Higher Order Rainbows
These are third order rainbows that form at the arc of 40°20′ around the sun. For one to see the higher order rainbow, one has to face the sun (Raymond & Alistair, 2001, p. 34). The visibility of the higher order rainbows as the third, fourth, and the fifth order rainbows are difficult. A fifth order rainbow was seen in the 19th century by Eleuthere Mascart and is perceived to lie between the primary and the secondary rainbows.
Moon Bows, Fogbows, and Coronas
The corona around the moon (Department of Atmospheric Sciences, 2011, p. 534)
The moon bows are rare lunar rainbows that occur due to the reflections of light during moonlight. Moon bows are visible when the sky is dark, and when the moon reflects maximum light; mostly when there is a full moon.
Moon bows occurs due to reflections and refractions caused by moisturized air by rain, mist, or moisture from a waterfall. Coronas are colored halos formed around the moon. They form because of moonlight diffractions caused by droplets in a cloud. Contrastingly, the fogbows are twilight rainbows, which are only visible during sunrise and sunset.
Sunrises and Sunsets
The sky gives red, orange, and yellow shades during sunrises and sunsets. The colors occur because the atmospheric constituents like gases, dust, and water vapor change the path of sunlight as it reaches the earth surface. Depending on the wavelength, light scatters filling the sky with yellow, red, and orange colors.
Sunset near a coast (Department of Atmospheric Sciences, 2011, p. 522)
Since red has the longest wavelength, there is a red visibility region when the sun is on the horizon (Lynch & Livingston, 2001, p. 14). Sunrises and sunsets are usually very colorful, and these colors are associated with the concepts of the rainbow, where the sun marks the main source of the light waves.
In conclusion, examination of the rainbow shows that refraction, refection, and dispersion of sunlight by raindrops makes rainbow to appear in the atmosphere. Furthermore, phenomena such as the beautiful moon bows, fogbows, coronas, sunrises, and the sunsets take place through the same process of the rainbow formation.
Specifically, dispersion of sunlight by dust and other particles in the atmosphere makes the sunsets to have a glorious color of red. Thus, these phenomena present some of the eloquent and beautiful creations caused by the refraction, reflection, and dispersion of sunlight.
Department of Atmospheric Sciences. (2011). Light, Color, and Atmospheric Optics. Retrieved from https://atmos.uw.edu/~hakim/101/Chapter19.pdf
Graham, L. F. (2007). The Rainbow Book. Berkeley: Shambhala Publications.
Greenler, R. (2000). Rainbows, Halos, and Glories. New York: Cambridge University Press.
Lynch, D. K., & Livingston, W. (2001). Color and Light in Nature (2nd ed.). New York: Cambridge University Press.
Minnaert, G. J., Lynch, D. K., & Livingston, W. (2006). The Nature of Light and Color in the Open Air. New York: Dover Publications.
Naylor, J., Lynch, D. K., & Livingston, W. (2002). Out of the Blue: A 24-Hour Skywatcher’s Guide. New York: Cambridge University Press.
Raymond, L., & Alistair, B. F. (2001). The Rainbow Bridge: rainbows in art, myth, and science. New York: Penn State Press.