Rainbow 1 - how do the colours of rainbow come about?

The simple answer is:
When light passes through a prism, it is dispersed into its composite different colours. For rainbow, when sunlight passes through the water droplets in the air, the colour get separated out.

Let's get into a bit more details for the preparation of the next few posts. When light, in fact any wave, meets a boundary, there is a change of direction depending on the speed of wave before and after the boundary. This is known as Snell's law which states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of velocities in the two media, or equivalent to the opposite ratio of the indices of refraction.

However, the refractive index for different colour of light is slightly different. The refractive index of blue is larger than that of yellow and that of red. Hence white light is refracted to slightly different path into a spectrum of colour. [list of refractive indices]

Sources:

Rainbow photo: http://www.flickr.com/photos/orvaratli/3605600578/
Prism dispersion: http://upload.wikimedia.org/wikipedia/commons/0/06/Prism_rainbow_schema.png
Snell's law" http://en.wikipedia.org/wiki/File:Snells_law2.svg

Rainbow 0 - introduction

Rainbow, both colourful and graceful, has been a main scene in the fairy stories. I am fascinated by the complexity of its beauty and the science behind it. This series of posts will progress from the kindergarten level understanding to post graduate research postulates. Quite a bit of mathematics will be required at the most advanced posts, but I shall provide the solution as we go.

introduction

As a 2010 New Year Resolution, I am going to write one post per day on science which does not appeal to common sense. If I am not able to do one today, I will do two tomorrow. At least, that is my resolution.

Common sense "consists of what people in common would agree on: that which they "sense" as their common natural understanding." [source] It is difficult to identify common sense. Common sense to you may not be the same to me. Anyway, what I will try to do is to identify some common interpretation or understanding of the natural phenomenon and try to explain in easy to understand language the actual science behind the phenomenon.

Am I qualified to do this? Not exactly. I was a high school Physics teacher for over 15 years before I switched to computing. I have written a couple of Physics textbooks back then. But without reading science as often as I was, I am now trying to learn new things together with you. I will only be using desktop research, so you can help by actually performing some of the experiments suggested to see if the observations actually support the theory.

Sometimes, I will also examine common sense and discuss the underlying science which may not have been well understood, while the common sense is scientifically correct.

This is a science oriented blog. I will not discuss any super-natural phenomenon. Do not ask me anything about magic, miracles, UFO, crop circles. I am an atheist and see the wonder of the Universe as it is.

The audience I will be addressing to are those with intellectual integrity, understand how logic and evidence work but not necessarily been educated with a science background. What I will demand from my audience are the ability to logically consider the evidences that are presented to them and evaluate whether the conclusion that is drawn based on the evidence is logical or not. Of course, science process does NOT prove anything as the proof offered in mathematics - science process is deductive. We seek generalisation of a set of similar observations. Propose a theory from which all the observations can be deducted. If there is any other possible outcome from the theory, i.e. derived cases, science demands that these cases be tested. If any one of these cases is found false, the theory will be discarded. This is a rather strict requirement. As we are unlikely to deal with cutting-edge science, I am not going to catch on those cases. If there is any controversy about a theory, I will stop and only report when consensus has been reached.

Hope you will enjoy the posts. Please leave comment which will be welcome and appreciated.