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July 2017 Archives

There is no denying it. I am middle-aged. The latest evidence is the progressive-lens glasses. I had tried to put off getting these for as long as possible (warning to you younger readers - they are not cheap!) but it was just getting too difficult without them.

We pretty-well take for granted good vision, but most of us can only enjoy really good vision by virtue of really good lens-making methods. In my case, while my vision was fine while at school, it became apparant rather quickly at university that mine wasn't as good as some of my friends. Sat at the back of one of the large lecture theatres I really couldn't kid myself that I had good eyesight. I couldn't read what was on the screen at the front, while others around me could. While my vision isn't so bad that I'd walk into things without my glasses, or be unable to read bus numbers on the front of buses*, I'm obviously short-sighted, meaning that I can happily look at things close to me (up to a couple of years ago, anyway), but far-off objects just can't be focused.  In terms of the physics of my eye, the focusing is mostly done by the cornea (which I don't have control over), and then a little by the lens. Muscles squash or stretch the lens in order to focus an image - a squashed lens is more powerful than the unsquashed lens and focuses the image closer to the lens**.  But in my case far away objects are focused in front of the retina and I can't stetch my lenses enough to bring the image into focus onto the retina. To correct this, I need diverging lenses in my glasses. 

But that's not all there is to it. I also have astigmatism, meaning that my corneas are not rotationally symmetric. They have different focal lengths for objects at different orientations. This means the lenses in my glasses need to have different strengths in different orientations.  

Unfortunately, now that's not all there is to it either. My lenses (the ones in my eyes) can't squash and stretch as much as they used to. This means that, despite being near-sighted, I now have trouble reading. For close-up work, I need a (slightly) converging lens. That's awkard - diverging for distance, converging for close work. Some people manage that by having two pairs of glasses, but since in my work I am constantly switching between close-up work and long-distance work and mid-distance work it would make for a logistic nightmare. Some people have bifocal lenses (including a small piece at the bottom for reading, but the rest of the lens the usual prescription). And some have progressive lenses. 

Progressives are rather neat and as a physicist I find it amazing that they work in practice, as well as in theory. Here the idea is that the power of the lens varies gradually from the top of the lens (which is diverging) to the bottom of the lens (slightly converging) - including the astigmatism correction as well. A consequence of an all-in-one lens is that not all the field of vision can be sharp at once. If I look out the sides of my glasses, things are a bit blurry. But one has to remember with vision that the brain is just as important as the eye. Although the blurriness is there (it was very apparant when I first put the glasses on) after a few weeks of wearing them I just don't notice it any more. Where I want to look is always in focus, and that's what counts. 

And that's made switching my gaze between books, computer screens, visitors in my office and the view of Mt Te Aroha from my office window a whole lot easier. 



*The same couldn't have been said of my father. He was very myopic (short sighted). His mother discovered this when my Dad was very young, following a conversation that went something like this.

Dad (about 4 years old, waiting with my Grandmother at a bus stop). Mummy, how do you know which bus to get on?

Granny: Because we can look at the timetable which tells us which number buses go where, and the times they go. So to go home from the city, we need a number 12 bus. 

Dad: Yes, but how do you know which bus is a number 12 bus?

Granny: Because it has a big '12' written on the front of it. 

Dad: Where?


**Contrast this to the way a camera focuses. With a camera, the lens is of fixed power - to focus the lens moves closer to or further from the sensor. 

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At the recent NZ Institute of Physics conference in Dunedin we heard about a wide range of different physics topics -measuring electrical forces; atomic frequency combs; why a highly gendered physics class is not a good thing and measuring forces with your phone. 

One very simple but thought-provoking presentation was by Tim Molteno - on sycamore seeds and their properties as little wind turbines. As far as I understood it, Tim's work here was a follow-up to what his son Linus had done as a science fair project. 

If, like me, you grew up amongst deciduous woodland, you'd have enjoyed playing with sycamore seed helicopters. They can fall very slowly indeed, giving the seed chance to drift in the wind away from its parent tree and get some chance of seeing some daylight when it germinates. It's pretty easy to measure the properties of a seed (it's weight, how fast it falls, and so on) at home and therefore do some basic wind-turbine calculations to see well the seed performs at slowing down the passing air (relative to the seed) and creating a lift force on the seed.  Tim and Linus have done this - and very well too - with a really careful consideration of uncertainties.  This last bit is important because the results show that the seeds are right on a fundamental limit (the "Betz limit") for their efficiency - that is, they are as efficient as a turbine could be.  

But they should not be - conventional wind turbine theory says that this limit can only be reached with a high tip speed ratio (the ratio of the tip speed of the blade and the wind speed). But the sycamore seed doesn't have a particularly high ratio and so its efficiency should be rather lower.  So one could say that these seeds are breaking the laws of physics.

Well, no, not quite.  What is more likely is that our understanding of turbine theory is lacking. Fluid flow can be very complicated indeed (ask an America's cup yacht designer) and some of the assumptions made in constructing the theory may not quite be correct.  Interestingly, the sycamore seed sits in the transition zone where the fluid flow isn't fast enough to be considered turbulent, but not really slow enough to be considered laminar and there might be some very complicated physics going on.

So, then, should make our electricity generating wind turbines the size and shape of sycamore seeds?  We would need a lot of them!

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To the idiots in Te Awamutu who thought it fun to shine a powerful blue laser at flight NZ5622 on its approach to Hamilton airport at 8.34 last night:

You may be surprised to learn that I would actually like to spend the rest of my life being able to see. And when I'm on an aircraft, I tend to feel slightly more secure if I know my pilots can see, too. Shining lasers at planes is not funny. You might think it's just a bit annoying to the pilots (ha ha!) - it is NOT.  Would you take someone's car and drain out the brake fluid for a joke? Or drop a concrete block onto a car from a motorway overbridge? Or put a bomb on a bus? Or shine your laser into your own eyes (a word of advice - as much as I like the idea of you doing it, DON'T.) You may think I exaggerate - but if you tried the same laser game in Australia or the US you may end up being treated by the law in the same way as would someone who put a bomb on the plane. Clever? No. 

You have been lucky in that I, and fellow passengers and the pilots as far as I know, are still able to see. Please take your laser, remove the battery, and hit it several times with a hammer. You might enjoy that just as much and it's a lot more productive.





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