A rainbow or a bianglala is an optical and meteorological phenomenon of a parallel, parallel color visible in the sky or any other medium. In the sky, the rainbow appears as a light bow with its tip leading to the horizon at a time of light rain. Rainbows can also be seen around the swift waterfall. 
Formation Edit Sunlight is polychromatic light (made up of many colors). The white color of the sun's light is actually a combination of different lights with different wavelengths. The human eye can perceive at least seven colors of sunlight, which will be seen in the rainbow: red, orange, yellow, green, blue, indigo, and violet.
The wavelength of this light forms a ribbon of parallel lines, each color nuanced with a color next to it. This band is called the color spectrum. In the color spectrum, the red line is always on one side and blue and violet on the other, and this is determined by the difference in wavelength. The rainbow is nothing but a large circular color spectrum arc that occurs due to the refraction of sunlight by the grains of water. When the sunlight passes through the water granules, it refracts as it penetrates the glass prism and exits into the rainbow's color spectrum. So in the water droplets, we've got different colors lined from one side to the other. Some of these colored lights are then reflected from the far side of the water droplets, back and out again from the water droplets. The light exits back from the water droplets in different directions, depending on the color. The colors in this rainbow are arranged with red at the top and purple at the bottom of the rainbow. 
The rainbow is seen as an arc from the surface of the earth because of the limited viewing angle of the eye, if the viewpoint in a high place eg from the airplane can be seen as a complete spectrum of color that is in the form of a circle. The rainbow can only be seen when it coincides with the sun shining, but from the opposite side of the observer.
The position of the observer must be between the sun and the water droplets with the sun behind the person. The sun, the observer's eye, and the center of the rainbow bow should be in a straight line.