Friday, 23 December 2011

A homemade rainbow

It has been a while since I have had time to post anything here. Well, rather late than never. Here is a photo of the sun's light falling through water in a wine glass and in the process being broken up into a spectrum of its constituent wavelengths:

And here is a closeup of a part of the rainbow:

Finally, a photo of a "real" rainbow I took some weeks ago:

My simple point-and-shoot camera is not the best tool with which to capture the rich colours. All seven, er, six, er... how many colours are there in a rainbow anyway? At school, we are usually taught that there are seven. Precisely seven, no more and no less, and the clever kids, none of whom have ever taken the time to look - really look - at a rainbow, can even name them for you: red, orange, yellow, green, blue, indigo and violet. Or something like that. I can never quite remember, and I don't think I have ever seen more than six clearly defined colours in a rainbow. Because the colours smoothly flow into one another, one can perhaps see more than seven. Or, depending on your culture and language, fewer than six.

Next time you see a rainbow, take note of how many colours you see. You may be surprised.

Edit, 24/12/2011:
There is a good article in Wikipedia about rainbows, that explains quite a lot about them, here.


  1. And why exactly are there TWO rainbows? And why are they exactly where they are and why is the outer one weaker? And why is blue on the INSIDE and red on the OUTSIDE?

    And why are they formed at all?

    And would they exist on mars and jupiter? And if so what would they look like?

    And are there rings of X-rays and Gamma-rays beyond the ultra violet? Why or why not?

    Please explain and illustrate your answer from first principles.

  2. All good questions. Some of them have occurred to me, but I made no mention of them here because I have no clue as to the answers.

    There is an article on rainbows at Wikipedia here:

    It does not tell me much either. For example, according to the diagram of a raindrop they have there, the blue should be on the outside of the rainbow rather than on the inside. So the more I read up on the issue the less I seem to know about it.

    But then, my point with the post was to raise questions rather than answer them. :-)

    As far as I know, X- and gamma rays do not penetrate far into the atmosphere, but if they did, I presume they would be invisible parts of the rainbow. If memory serves, if you make a mini-rainbow with a prism, and then put a thermometer just beyond the red, it indicates a rise in temperature there because of the invisible infrared band.

    I presume rainbows would exist on Jupiter, at least in parts of its atmosphere that are at suitable temperature and pressure for water droplets to exist. I would guess droplets of other liquids would also work.

    Mars might be a bit different, since only ice crystals would exist in its atmosphere - perhaps one would see phenomena like sun dogs and rings around the sun there?

    All this just goes to show how intricate the cosmos in which we find ourselves is. Even common phenomena, with which we are well acquainted, are full of fascinating secrets.

  3. After a good night's sleep I have no reread the Wiki article, and it makes much more sense now. It answers many of the questions you ask above.