Light

The nature of matter

Few concepts in Physics generate wonder quite like Quantum Theory. You only need to look at the shelves in the Popular Science section of a bookstore for evidence. Yet (once again) in schools we play down this sense of wonder. I used to think this wasn’t done deliberately but now I’m beginning to learn that there was once a school of thought that believed in doing exactly that, particularly for Science (more on that later).

Anyways, one of the most incredible ideas in Quantum Theory concerns the nature of matter itself – is it a particle or is it a wave?
For light, we can prove that it’s both (ridiculous though that may sound) and indeed students are expected to know the demonstrations which verify both. There is however no suggestion anywhere in either the textbooks (that I have come across) or in the syllabus that there is anything slightly disturbing in this. There was a single question on the exam paper once which asked why was Quantum Theory considered revolutionary, but that was it. No other reference to what is one of the greatest mysteries of science; how can something be both a particle and a wave. Why do I seem to be the only one who feels so frustrated by this?

So in an attempt to pass on some of this sense of wonder for the microscopic world, I put together the following set of demonstrations for my sixth years on the last day of term. It’s about 22 minutes long so is in two parts. Forgive its amateur appearance.

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Is it a particle or is it a wave?

Sometimes I think I’d gladly be locked up in a dungeon ten fathoms below ground, if in return I could find out one thing: What is light?
Galileo, from the play Life of Galileo, by Bertolt Brecht

 The single greatest source of debate among physicists in the early decades of the last century was to do with the nature of light. Come to think of it, this concept has probably caused more angst than any other to scientists and philosophers right back to the ancient Greeks.
To take just one aspect; we can prove that light is a particle (via the photoelectric effect) and we can prove that light is a wave (via interference, or the famous ‘double slit’ experiment) yet particles and waves are two completely different phenomena. Particles are ‘things’ and are therefore supposed to be localised in space and have mass. And while there are  different varieties of waves, they are not supposed to be ‘in one place’ or have mass.
So what gives?


 

Answer: nobody knows. To this day there are different interpretations, but none that is accepted by all. The YouTube clip below shows some of the world’s greatest physicists coming together for one of a series of conferences to try to make sense of it all back in the 1920’s. Needless to say they did not reach a consensus. There is wonderful book called QUANTUM which describes in great detail the history of this debate at the beginning of the last century. See here  for a previous post on the book itself.

Now in leaving cert physics we need to know the evidence for light being both a particle and a wave. But there is room in the syllabus or any of the textbooks that I have come across to highlight the bizarre nature of this. It lies at the heart of one of the greatest problems scientists have ever faced, and our response is to simply pretend that there is nothing of note here.

It’s simply not good enough.

Mr Devitt teaches us about Light

Jerome Devitt (History Teacher) runs a transition-year module on Light and Sound for theatres, and kindly agreed to allow us in to watch him teach one of his lessons.

Not all of it comes out great on video, but it should still prove very usefull for Senior Phyics classes.

If you would like to play with a virtual version of this, where you can control the postion and intensity of the lights, click here.

Thanks Jerome