Earthquakes and Polarized light

By Marcus Wilson 07/09/2010 1

I had to get up early last Saturday to catch my flight back home from Dunedin to Hamilton, via Christchurch. My fears of sleeping through the alarm clock proved irrelevant as I was supplied with a rather more violent variety courtesy of plate tectonics underneath Christchurch. (Dunedin is a long way from Christchurch – given the extent of the shake in Dunedin all I can think is that it must have been a nightmare for those close by). 

I turned up at the airport with my PhD student on time to discover, really to no great surprise, that we weren’t going to be flying to Christchurch that morning. Instead, we got put on a flight to Wellington, and had several hours to wait there before a flight with space on to Hamilton.

But there’s a lot to do in Wellington for a few hours, so my PhD student and I went off to Te Papa for a look around. One of the things we had a look at was the colossal squid. As part of that display, there is a short 3D film, using polarizing glasses to view it.  These glasses work by selecting just one polarization for each eye – so that each eye sees a slightly different view, which the brain interprets as a 3D image. Two pictures are projected onto the screen, in two different polarizations. Being physicists, we duly played with a couple of pairs of glasses, putting a left-eye lens over a right-eye lens and observing that everything went dark. 

So what is polarized light?  Light is an electromagnetic wave – it has a magnetic field and an electric field. We are probably more familiar with magnetic fields – it’s what causes a compass to work. Electric fields are associated with electric charges – we experience them with things like static electricity. The electric field and magnetic field are always perpendicular to each other, and perpendicular to the direction of travel of the wave. So if a wave is coming towards you, the electric field might be pointing straight up, and the magnetic horizontally, or the electric might be pointing horizontally and the magnetic vertically, or both at some other angle (so long as the two are 90 degrees apart).  The polarizing glasses probably contain a material that is strongly directional – e.g. chain like structures orientated in a particular direction, that allow the light to pass if it has (say) the electric field aligned vertically, but absorbs the light if the field is aligned horizontally. Thus they are selective to a signle polarization.

A similar 3d effect can be achieved with red and green filters, but the disadvantage is that you don’t see the film in colour.  (The colour of light depends on the  frequency of the wave’s oscillation, and is independent of polarization).  But the 3d film using polarizers isn’t ‘perfect’, you also need to make sure the screen doesn’t destroy the polarization of the projected light, so usually you need to invest in a ‘silver’ screen too.

Quite fun, but whatever way you view 3d films you still need to wear glasses which make them still a bit of a strange experience.

One Response to “Earthquakes and Polarized light”

  • The reminds me of an experiment that we did in sixth form Physics. When you have two polarising filters set at right angles to each other, no light gets through. If you take a third polarising filter and insert it at an angle between the other two filters, then some light does get through. So introducing a filter (which by definition blocks light), allows more light through. Cool, huh ?

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