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Physics of Rainbow Formation: Critical Essay

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Rainbows are known for presenting themselves as a varied and fascinating phenomenon to the observers. The explanation of the formation of rainbows in optics usually incorporates the discussion of light rays through their bunching and spherical drop for certain angles of deflection, which corresponds to both the first-order and the second-order rainbows. Notably, rainbows have generated multiple myths and legends because of their unique and equally colorful display. In most cases, rainbows are seen when light from the sun passes through raindrops. This leads to the creation of a beautiful phenomenon that appeals to the observers. Significant research has been conducted by experts to establish the finer details concerning the formation of both primary and secondary rainbows. This paper provides an in-depth explanation of a rainbow is formed. This paper further explains how secondary rainbows are formed, as often witnessed when double rainbows are formed.

Suzuki and Suzuki (2010) refer to a rainbow as a visual spectacle whose explanation can be made through the application of the wave nature of light. A rainbow usually consists of both a primary and a secondary rainbow. The primary rainbow, as noted by Suzuki and Suzuki, is formed by a single internal reflection and two refractions that occur inside a rain droplet. On the other hand, the secondary rainbow usually appears outside the primary rainbow. It is formed when two internal reflections and two refractions occur inside the spherical droplet of water. Suzuki and Suzuki further state that the colors seen on a secondary rainbow are often reversed from the primary rainbow. However, the secondary rainbow is twice as broad as the primary rainbow (Matokwe et al., 2016).

According to Suzuki and Suzuki, the dark band that appears between two different rainbows is called the region of negligible scattering. Rainbow polarization occurs as a result of internal reflection. The back surface of the drop that is found closer to the Brewster angle is usually stricken back by the rays such that nearly all the reflected light becomes perpendicularly polarized towards the incidence plane. As established by Corradi (2016), rainbows do not either form any segment that is greater than a semicircle or a full circle. During sunrise and sunset, the segment becomes the largest, while the circle becomes smaller. However, as the rise rises higher, the segment becomes smaller, while the circle, on the other hand, becomes smaller. Usually, there are never three or more rainbows at one particular time. Instead, each of the two rainbows formed has three colors, and the colors are similar in the two rainbows, and their number is also the same. In the outer rainbow, however, the colors are usually painted, and the positions of the colors are reversed.

Rainbows that occur as a result of the reflection and refraction of light from the sun appear in the section of the sky that is directly opposite the location of the sun. Although rainbows can be full circles, observers can only see an arc that is formed through the illumination of droplets that appear above the ground (Trevino, 2019). These droplets are also centered on a line that appears from the sun to the eye of the observer. In primary rainbows, for example, the arc usually appears red within the outer part of the rainbow, while, on the other hand, the inner side of the rainbow seems to be violet. For double or secondary rainbows, a second arc usually appears outside the primary arc (Corradi, 2016). This leads to a reversed order of the colors such that the red color appears on the arc’s inner side. This occurs when the light from the sun undergoes a double reflection inside the water droplet before it leaves the droplet.

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When light from the sun passes through raindrops, Trevino (2019) states that the light refracts or bends as it passed through the droplets. The sunlight then causes a reflection off the sides of the drops. As further posited by Trevino, this occurs because the density of the water is more as compared to that of the air within its surroundings. When the light from the sun exits the water droplets, it separates into multiple wavelengths. Notably, visible light consists of numerous wavelengths, so each wavelength becomes visible as a different color, such as orange, red, green, yellow, indigo, blue, and violet. For example, Trevino states that red light usually bends at a different angle as compared to violet light.

The beautiful colors on the rainbow are a result of the variation of the reaction index of water with wavelength. The visible interval of light, as noted by Suzuki and Suzuki, lies between 400 and 700 nm, such that n=1.331 for the red light, while for the blue light n=1.343, and the violet color n=1.344.

In secondary rainbows, there are two internal reflections and two refractions that occur in the path of the light. When there is a double reflection of light rays inside a water drop, secondary rainbows are formed. This occurs in four steps repeated reflection and refraction. As a result, the intensity of light becomes reduced during secondary reflection. This makes the primary reflection to be brighter compared to the secondary reflection. Moreover, the order of the colors in the rainbow becomes reversed, particularly in the instance of secondary rainbows.

In conclusion, rainbows are a varied and fascinating phenomenon that attracts the attention of observers because of their appearance and beauty. The primary and the secondary rainbow are formed as a result of the reflection and refraction of light from the sun, in the case of natural rainbows. The formation of rainbows occurs as a result of the refraction and the reflection of sunlight from the sky. Even though rainbows may be formed as full circles, the only visible part of the rainbow to observers is an arc, which is usually seen above the ground. This arc, for instance, in primary rainbows, appears on its outer part, while the inner side becomes violet. Significant research has been conducted to establish how primary and secondary rainbows are independently formed. There is a need for more research to help in determining the formation and structure of primary and secondary rainbows.


  1. Corradi, M. (2016). “A Short History of the Rainbow”. Lettera Matematica, Vol. 4, No. 1, pp.49-57. [Accessed 19 Nov. 2019].
  2. Ježek, F., and Buchtová, H. (2015). “The Effect of Vacuum Packaging on Physicochemical Changes in Rainbow Trout (Oncorhynchus Mykiss) During Cold Storage”. Acta Veterinaria Brno, Vol. 83, No. 10, pp.51-58. [Accessed 19 Nov. 2019].
  3. Können, G.P. (2017). “Rainbows, Halos, Coronas, and Glories: Beautiful Sources of Information”. Bulletin of the American Meteorological Society, Vol. 98, No. 3, pp.485-494. [Accessed 19 Nov. 2019].
  4. Matokwe, T.B., Jung, H.Y., Kang, K.H., Do Jeong, H. and Kim, J.K. (2016). “Characterization of Acidogenesis Occurring on Rainbow Trout (Oncorhynchus Mykiss) Sludge by Indigenous Alcaligenes Faecalis”. Biotechnology and bioprocess engineering, Vol. 21, No. 6, pp.794-803. [Accessed 19 Nov. 2019].
  5. Selmke, M. and Selmke, S. (2018). “Revisiting the Round Bottom Flask Rainbow Experiment”. American Journal of Physics, Vol. 86, No. 1, pp.14-21. [Accessed 19 Nov. 2019].
  6. Suzuki, M. and Suzuki, I.S. (2010). “Physics of Rainbow”. [Accessed 19 Nov. 2019].
  7. Trevino, J. (2019). “How Is a Rainbow Formed?”. Retrieved 19 November 2019, from [Accessed 19 Nov. 2019].
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Physics of Rainbow Formation: Critical Essay. (2023, October 11). Edubirdie. Retrieved February 21, 2024, from
“Physics of Rainbow Formation: Critical Essay.” Edubirdie, 11 Oct. 2023,
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