Why is the rainbow coloured?
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth’s atmosphere. They take the form of a multicoloured arc, with red on the outer part of the arch and violet on the inner section of the arch. A rainbow spans a continuous spectrum of colours; the discrete bands are an artefact of human colour vision. The most commonly cited and remembered sequence, in English, is Newton’s sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G. Biv). Rainbows can be caused by other forms of water than rain, including mist, spray, and dew.
Prismatic refraction of light. If you shine light through a pyramid shaped crystal you will get a separation of the different colours of light projected from out the other side. Clear light we are looking through only appears clear to our eyes and contains all the colours of the rainbow. Droplets of vapour in the air above the atmosphere act as prisms separating the light coming at it from a particular angle. See if you can find an elementary book on physics and/or optics which explains things well. An encyclopedia may help. If you ever watched the Walt Disney movie “Pollyanna” there is a scene in which Pollyanna and Jimmy Bean string a bunch of crystals across a window resulting in a rainbow like projection on the walls and contents of the room.
The mechanics of rainbows have been studied since ancient times. The Greek philosophers were aware of the role of reflection in forming a rainbow, and had some understanding of the role of refraction. In the 13th century, scientists produced theories on rainbow formation, and in the 17th century, Rene Descartes sketched out the conditions required to observe a rainbow. We see rainbows because of the geometry of raindrops. When the sun shines from behind us into the rain, incident rays of light enter the drop and are refracted inwards. They are reflected from the back surface of the raindrop, and refracted again as they exit the raindrop and return to our eyes. Refraction is responsible for splitting the sunlight into its component colors.
