Sample Astronomy Paper on Rainbows and Blue Skies

Rainbows and Blue Skies

Why is the sky blue?

One of the reasons for the sky’s blue appearance is Rayleigh scattering. In 1871, Lord Rayleigh came up with a formula giving a description of a set of interaction between wavelengths of visible light and atmospheric particles such as oxygen, nitrogen, and dust particles. Rayleigh stated that most of the atmospheric particles, to be specific, oxygen and nitrogen molecules, are much smaller than the wavelengths of the radiation that strikes them. Rayleigh’s system goes ahead to show that in such interactions, there is an inverse variation of the intensity of scattered light with the fourth powers of its wavelength. According to Rayleigh, blue and violet with shorter wavelengths tend to scatter a lot more than longer wavelengths such as red, orange, and yellow when they meet relatively smaller particles such as oxygen and nitrogen particles (Long et al 2). As such, scattered light disperses in all directions making the sky be saturated with one color, which is blue. It is important to note that as visible light passes through the atmospheric space, the already mentioned longer wavelengths such as red, yellow, and orange pass straight through and are neither scattered nor affected in any way. On the other hand, for the shorter wavelengths specifically blue, they are first absorbed by oxygen and nitrogen molecules and then the blue light is scattered or radiated in different directions all around the sky. This is why most of the time, whichever the direction viewed by the human eye; the scattered blue light reaches the eye leading to the conclusion that the sky is blue. However, distant places such as the horizon appear pale blue because the scattered blue light has to pass a longer distance and through more air before reaching the sky, and thus, some of the blue light is scattered away again in other directions resulting in less blue light reaching the eyes. There has been concern about only blue wavelengths being scattered yet other wavelengths such as red, orange, and yellow wavelengths are not scattered. Apart from blue wavelengths being shorter than red, orange, and yellow wavelengths, they are more energetic and thus scatter more (Long et al 2). Blue wavelengths have higher energy and thus can pin-pong around more, and also because of their interaction with particles available in the atmosphere. One fact is that if the earth had no atmosphere or if the atmosphere had no particles such as gas molecules, the human eye would see wavelengths of visible light as they emanate from the sun. Another argument for the sky’s blue appearance is based on the human eye and brain’s interpretation of certain combinations of wavelengths as a single discrete color. In essence, the blue-violet wavelengths of the visible light are often interpreted by the human visual sense as a mixture of blue and white light, and this is why people always see the sky as light blue in color.

What causes rainbows?

Rainbows are without a doubt among the most beautiful displays of nature, although there have been several explanations and beliefs about its creation form different ancient cultures. One fact that people ought to understand is that a rainbow does neither in existence in a particular place nor is it a real substance. Instead, it is considered an optical phenomenon that appears or is seen in the right atmospheric and sunlight conditions. One of the conditions for the formation of rainbows is that there must be water droplets floating in the atmosphere, and this is why rainbows are commonly seen after rains (Topdemir 75). Another condition is that clouds must be cleared away from the sun to result in the formation of rainbows in the opposite direction of the sun. When sunlight shines or passes through water droplets, it if forced to bend, a process known as refraction, because of the slower speed at which it travels in water than in air. The fact that water is denser than air results in the significant reduction of the speed of light in water. As sunlight rays bend, they are bounced off the back of water droplets in the atmosphere and trace their way back to where they originated. This is illustrated below:

Fig. 1: Formation of Rainbow


A rainbow has seven colors namely red, orange, yellow, green, blue, indigo, and violet and questions have been raised about the origin of these colors during the formation of rainbows. As mentioned earlier, sunlight has several wavelengths, some of which bend more than others when sunlight passes through water droplets. One of the wavelengths of sunlight that bends the most is violet, which is the shortest wavelength of visible light, and this is the reason for its appearance on the inner edge of a rainbow. The wavelength that bends the least is red, which is also considered the longest wavelength of visible light, and this is the primary reasons for its appearance on the outermost edge of the rainbow (Topdemir 75). At times, another dimmer rainbow with a reversed order of colors can be seen higher in the sky, what is often referred to as a secondary rainbow. The latter often forms because of additional reflection of sunlight through water droplets, and it is often visible in the presence of dark clouds behind it.

Why are clouds white

Clouds are large collections of very tiny droplets of either water or crystals of ice, and the fact that they float in the air is because the water or ice droplets forming clouds are very small and light. As the case of the sky and the rainbow, the white color of clouds is attributed to the different wavelengths of visible light and as already mentioned, visible light travels in wavelengths that include red, orange, yellow, green, blue, indigo, and violet. When these wavelengths hit the water droplets or ice crystals that form clouds, they are scattered, and they combine to produce white light and this leads to the conclusion that clouds are white in color (Ahrens 48). Unlike oxygen and nitrogen molecules found in the atmosphere, the molecules of water droplets and ice crystals are large enough, and thus, have the capability to scatter the light of the seven wavelengths. In fact, none of the wavelengths whether short or long is capable of passing through ice crystals and water droplets floating in the air. At times, especially when rains are almost, clouds appear dark because they are often thick such that they do not allow light to pass through hence the dark color of clouds when rains are almost. Besides, the shadow of other clouds can result in the dark color of others.

Works Cited

Ahrens, C. Donald. Meteorology today: an introduction to weather, climate, and the environment. Cengage Learning, 2012.

Long, C. N., D. W. Slater, and Tim P. Tooman. Total sky imager model 880 status and testing results. Richland, Wash, USA: Pacific Northwest National Laboratory, 2001.

Topdemir, Hüseyin Gazi. “Kamal al-Din al-Farisi’s explanation of the rainbow.” Humanity & Social Sciences Journal 2.1 (2007): 75-85.