Because the sun is spherical, its light reflection is round. The visual limit of the human eye cannot surpass the horizon. Because of that, only half of this form can we see. Maybe, if the sun is a cube, the resulting rainbow is in a rectangular shape. Who knows? So, actually rainbow does not even exist. It is not an object and we cannot touch it. In addition to the semi-circular shape in the sky, rainbows can also result in waterfalls, in fog, dew, waves and wherever light meets water droplets and seen from angle of 42 °. In Greek and Roman times, the rainbow is believed to be a path to an eternal place. Some consider a pot with gold waiting at the end of each rainbow. In fact, some still believe.
History of the study of the rainbow is said to have originated from Greek philosophers, Aristotle. He submitted some theories about the rainbow. However, there are many shortcomings and can not be proven. A Greek poet, Homer, argues that the rainbow consists of one color, namely purple. It does seem illogical, but that is his opinion. Later, the philosopher Xenophanes add two more colors, to purple, yellow green and red. Around the 10th century, Arab physicists, Ibn Haytham gave a scientific explanation for the rainbow phenomenon. In his book, 'Maqala fi al-Hala wa Qaws Quzah', he explained that the formation of a rainbow as a picture formed on a curved mirror. He managed to find that sunlight as a source which is reflected when meeting any point in the mirror. So, from a distant source of sunlight, with the eyes of the audience as one points in curved mirrors and clouds as the surface of the reflection, the rainbow is produced.
However, he could not confirm this theory. But, the Persian scientist, Ibn Sina anyway, give an alternative explanation. According to him, the clouds only serves as a background and does not play a role in the formation of the rainbow. Ibn Sina asserted that the formation colors are only subjective to the eye. He agreed a lot with Aristotle concerning the rainbow. Then, in the 13th century, Persian astronomer, Qutb al-Din Al-Shirazi with his students, Kamal al-Din Al-Farisi managed to provide a more satisfactory explanation mathematically. They found that sunlight is biased twice by water droplets and light reflections occur between these two refractions.
Al-Farisi made a model with spherical glass bowl filled with water to prove this theory. Through that experiment, the rainbow is believed to be the result of refraction and reflection of light. Meanwhile in Europe, Ibnu Haytham's book is being translated into Latin and became one of the main sources in the study about the rainbow. In the Renaissance, it was agreed to four colors namely red, yellow, green and blue. In the 17th century, however, purple is included again. In 1637, René Descartes detailing again the studies on refraction and reflection light in water droplets. And finally, in 1666, Sir Isaac Newton added two more colors, namely orange and indigo, to the seven colors we know so far. The rainbow is indeed a very beautiful natural phenomenon. It depends if you want to consider it as a sign of good fortune. But, from a scientific point of view, the rainbow is just a natural occurrence as is the case with other incidents such as an eclipse, earthquake or tsunami.
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