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January 2015 Archives

"Doctor, Doctor, I keep seeing spots before my eyes"

"Have you ever seen an optician?"

"No, just spots".

The concept of seeing an optician floating across my field of view is a scary one indeed. However, the concept of seeing spots doing the same is one I'm coming to terms with. 

I had a talk to an opthamologist about this last week, as part of an eye check-up. He was very good, I have to say, and we discussed in detail some optical physics, particularly with regard to the astigmatism in my right eye (and why no pair of glasses ever seems quite right).  He also reassured me that seeing floaters is nothing, in itself, to be worried about. It's basically a sign of getting old. How nice. He did though talk about signs of a detached retina to look out for (pun intented) - and did some more extensive than usual examination. 

So what are those floaty things I see? To use a technical biological phrase, they are small lumps of rubbish that are floating around in the vitreous humour of the eye. They are real things - not an illusion - although I don't 'see' them in the conventional manner that I would see other objects. 

The eye is there to look at things outside it. Its lens focuses light from objects onto the retina, where light sensitive cells convert the image to electrical signals that are interpreted by the brain. But given that the floaters are actually between the lens and the eye, how am I seeing them?

There are a couple of phenomena going on. First of all, a floater can cast a small shadow onto the retina. You can see this effect by using a lens to put an image of something (e.g. the scene outside) onto a piece of card, and then put something between the lens and the image. Some of the light can't get to the card, and so part of the image is shadowed. The appearence of the shadow depends on how close the object is to the card - if its right by the lens there will be very little effect - but if close to the card there'll be a tight, well-defiined shadow. My experience is that these spots are definitely most noticable in bright conditions - presumably because the shadows on the retina then appear in much greater contrast than under dull conditions. 

Secondly, however, they can bend the light. Their refractive index will be different from that of the vitreous humour, and therefore when a light ray hits a floater it will bend, a little. The consequence is a defocusing of a little bit of the image, which wil be visible. If the floater stayed still, it would probably barely be noticable, but when it moves, the little bit of bluriness moves with it, and the brain picks up the movement rather effectively. 

The most interesting thing to me is that it just isn't possible to look at these things. When I try, my eyes move, and consequently these bits of rubbish flit out of view. Rather like quantum phenomena, you can't observe them without changing where they are and where they are moving to.  

 

 

 

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Alison has drawn my attention to this video. It demonstrates vibrational modes of a square plate by using sand. At certain frequencies, there are well defined modes of oscillation, in which parts of the plate 'nodal lines' are stationary. The sand will find its way to these parts and trace out some lovely pictures. 

Vibrational modes are often illustrated through waves on a guitar string. Here, the string is held stationary at both ends, but is free to vibrate elsewhere. There is a fundamental frequency of oscillation, where the distance between the ends of the string is half of a wavelength (this ensures the displacement of both ends of the string is zero since they are clamped).  Since wavelength is related to frequency (frequency = speed/wavelength) that means if the wavelength is 2 L where L is the distance between the ends of the string, we have frequency = speed/2L.  

But that's not the only possible mode. Another one would have L equal to a whole wavelength (equals two half wavelengths). Or one-and-a-half wavelengths (equals three half-wavelengths.) This gives us, rather neatly, frequency = n speed/2L, where n is an integer. We see that our 'harmonics' are just integer multiples of the fundamental frequency. Rather neat.

However, if you look at the frequencies given in the video, they appear to be all over the place. I challenge you to pull out the relationships between these (I've tried). There are a few reasons why the case shown on the video is considerably more complicated than the waves on the string. 

1. The boundary conditions. The edges of the plate aren't clamped in place. This makes it less straightforward to define the modes geometrically. 

2. The plate is square, giving rise to 'degeneracy' in the modes. This term refers to two or more distinct modes having the same frequency. You can see it rather well with the 4129 Hz mode. Basically, there are horizontal stripes shown. But equally, with the same frequency, you could get vertical stripes. Why don't the two occur together? They do. You can see the effect of having a little bit of vertical stripe most clearly at the far end of the plate, where the pattern becomes more square-like. Also, with a square, you can get two completely different types of mode with the same frequency. This occurs because what matters are the sums squares of pairs of integers. Broadly speaking (at least for a square clamped on the edges, which I must point out this ISN"T), our modes follow the relationship:

f = C sqrt(n^2 + m^2)

where C is a constant, 'sqrt' means square-root, and n^2 is n-squared. So, for example, not only is 50 equal to 5-squared plus 5-squared, it is also equal to 1-squared plus 7-squared (or 7-squared plus 1-squared). This gives us three  modes all competing to appear at exactly the same time. What happens then isn't easy to tell. 

3. Non-linear effects. This a physicist's code-word for 'it's all too difficult'. That's not quite true - arguably most of the interesting physics research happening in the world is looking at non-linear effects. What this really means is that, if A and B are both solutions of a problem, then some combination of A and B is NOT a solution. A lot of physics IS linear - Maxwell's equations in a vacuum is a good example - but a whole lot isn't. With waves, the speed of the wave usually depends on frequency (i.e. is not constant) which means we lose the nice, integer-multiple relationship of our waves-on-a-string mode.

So, enjoy the video for what it is, and don't try to analyze it TOO closely. 

 

 

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I'm sure many readers will know that one of the hats I wear is the treasurer of the New Zealand Institute of Physics. NZIP is the professional organization for physicists within New Zealand. Its aim is to promote the interests of physics and physicists, at all levels, within the country. In addition to counting the beans, the role comes with a position on the council, and therefore I have a significant responsibility for looking after the institution.

With that in mind, I had the dubious pleasure of travelling to Wellington shortly before Christmas to represent NZIP at a meeting of council members of New Zealands 'science' societies at the Royal Society of New Zealand. I use 'science' in a very broad context here.  It was one of those ten-degrees-with-horizontal-rain summer days in the captial*, though that didn't matter too much as the day was spent inside RSNZ's rather nice new building. The day was actually very useful, as we talked through some of the common issues facing our science societies. 

One clear issue that many societies are facing (NZIP included) is dwindling membership. With dwindling membership comes dwindling income, making it harder for the society to do useful things. A great many socieities can't afford paid staff and so run on volunteers who necessarily need to prioritize their time elsewere.  Dwindling income basically means loss of services that can be offered to members, such as travel grants, teaching materials, careers advice, prizes and so forth.  It's a vicious cycle. 

But it's not all bleak, so long as we are prepared to accept the message that is coming from the research in this area. A recent report from the Australasian Society of Association Executives [which I'm afraid I can't find openly on their website, so no link I'm afraid - you might have to pay membership fees for it ;-)  ] talks about the changing face of membership. The report is rather pessimistically and not entirely accurately  titled 'Membership is Dead'. It talks about the difference in expectations that a 'Generation Y' person has from the Baby-boomer (I so hate those stereotyping terms - but the report uses them). What particularly got my attention is that the very things that Generation Y values [clearly defined value to them, responsiveness (which means hours or minutes or instant, not days or weeks), innovations, accessibility] are things that baby-boomer-dominated councils see as low priority, because they themselves don't value them. In other words, the expectiations of Generation Y and Council members when it comes to what a science society is and does are vastly different. A Baby-boomer may happily pay their membership fees year-on-year because they feel it's part of their duty as a professional to belong - a Y-er is less likely to take that generous line. If they can't see the value to them, they don't cough up. (For the record, I'm an X-er). It encourages institutions to work hard and getting younger people onto councils, by actively targeting undergraduate students, for example by giving them opportunities to assist with conference organization, website development and maintenance, tweeting on behalf of the society, etc. Then step back and let the younger people run it in a way that the younger people (=future members) want it run. 

So membership isn't actually dead. Instead, we just need to accept that 'membership' is going to mean something different to our younger people and adapt to account for it. Because, if we don't, our societies will dwindle away, to be replaced by something rather different. 

*Maybe I'm a bit harsh here. On our return from the South Island after our Christmas Holiday, the ferry Aratere (with full a complement of screws (propellors) and a fully-functional electrical system) took us across a flat Cook Strait and into a beautifully calm and sun-kissed Wellington harbour. As they say, you can't beat Wellington on a fine day...

 

 

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