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Posts Tagged light

Seeing circular polarization Marcus Wilson Nov 22

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Physicsworld magazine is doing a ‘special feature’ this month on animal superheroes – those with rather unusual physical abilities.

The best of the lot (in my subjective opinion) is the featured-on-the-cover mantis shrimp. Not because of its ‘dactyl clubs’ that can produce a force of 700 N, but because of its eyesight.

The mantis shrimp can see circularly polarized light - something that no other animal is known to do. Polarization describes how the electric and magnetic fields in the light wave are oriented. For example, a horizontally-travelling light wave (say in the x- direction) might have its electric field pointing in the z-direction (vertically) and the magnetic field in the negative y direction. In an electromagnetic wave, the electric field, magnetic field and direction of travel are all mutually perpendicular. We could call that a vertical, plane polarization.

In circular polarization, the electric field moves in a corkscrew-like shape as the wave travels. The corkscrew can spiral one of two ways – hence there are two distinct polarizations which we call left-handed and right-handed. The mantis shrimp can distinguish between the two. It does this by using its own version of a quarter-wave plate – made of a birefringent material – one that has a different refractive index in different directions. That converts a circular polarization to a linear polarization, which it detects via more conventional methods. (There are several animals that can ‘see’ linear polarization – bees are a famous example. There are plenty that don’t distinguish one  polarization from another at all, such as humans.)

The mysterious question is why? Bees use linear polarization to assist navigation (light from the sky is linearly polarized), but what use is distinguishing left-handed and right-handed circular polarizations to a shrimp? There’s a cool research question for someone’s PhD thesis.

 

Pinhole cameras and eclipses Marcus Wilson Nov 15

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Well, the eclipse yesterday was fun. There were enough patches of sky between the clouds to get some good views. I was pleased that the pinhole cameras I made out of miscellaneous cardboard tubes, tins, paper and tinfoil worked really well. Also, the trees around the front of the sciences building gave some nice natural pinholes as the sunlight worked it’s way through the gaps between the foliage – we could see lots of crescents projected onto the wall of the building. Not something you see everyday.

The trick with the pinhole camera is to get the combination of length between pinhole and screen and size of pinhole correct. (Basically – the f-number in photography-speak) A long length means a larger image – but also a fainter one. To increase the brightness, we need to let more light through (a bigger pinhole) but the drawback of this is that it blurs the image. It takes a bit of experimenting – best done well before the eclipse that you want to see.

On the subject of which…if you live in New Zealand…you don’t have a lot of opportunity for a while. We northerners get an iddy-biddy eclipse next May (10th) – sorry Mainlanders – you miss out – and then it’s nothing for ages before we get a few more feeble partials in the 2020s. BUT, as I said earlier, it’s then non-stop eclipse mayhem from 2028, with THREE total and THREE annular eclipses before 2045, for those of us who are still alive to see them. Details are all here courtesy of RASNZ.

There are a few videos up already from the Cairns region – here’s one. However, video does not do an eclipse justice, partly because of the difficulty in video capturing parts of the corona at different luminances simultaneously. If you want to see the fainter, whispy stuff at the far edge of the corona, you end up well overexposing the brighter area nearer the moon.  The naked eye does a far better job of capturing the totality phase than a camera. 

http://www.youtube.com/watch?v=CTbIufApsSk

I note a fair amount of pink on the video – this is the chromosphere – a thin, cooler area of the sun, between the photosphere (the bright yellow bit that we normally see) and the corona.

 

 

 

 

Pepper’s Ghost Marcus Wilson Nov 01

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 Have a good look at the photo. The pretty rhododendron to the left of the chair looks a bit odd. That’s because it’s a ghost shrub. No, our garden isn’t haunted, and neither have I doctored the photo; it’s an example of Pepper’s Ghost – an illusion caused by reflections. The bush in question is off to the right, out of frame, and the camera is seeing its reflection in the window. Because the bush is well lit, but the background isn’t, it appears to be ‘real’. The effect looked even more stunning with polarizing sunglasses on.

 

 

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Distant galaxies and hobbits Marcus Wilson Oct 01

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I haven’t read ALL of Tolkien’s work, but I suspect space-travelling hobbits don’t feature anywhere. However, what do feature are hole-dwelling hobbits, and I had the fun of seeing their holes in the countryside near Matamata yesterday. The original set for Lord of the Rings was mostly removed after filming, and rebuilt for the filming of the Hobbit trilogy.  (Trilogy? Since when was The Hobbit a trilogy? This is just milking money out of Tolkien fans, isn’t it?) But this time the set will remain, for all to see, for an appropriate fee of course. It certainly was fun to have a look around – what made it was the commentary provided by our excellent guide.

One of the fascinating things pointed out was the perspective tricks that were used. For The Hobbit, there are three different versions of some of the holes.  One, a ‘large’ version, appropriate for a normal-sized actor, dressed as a hobbit, to walk through. One, a smaller version, to make the dwarfs look bigger than the hobbits. And another, an even smaller version, to make Gandalf look bigger than the dwarfs. And the three had to be identical.

And then there are the perspective tricks. To make the view look like it is over a longer distance, the more distant holes are of smaller size than the nearer ones. On a 2d movie it works – your mind interprets what you see as being of equal-sized holes spread over a larger distance. But being there in 3d you see it more as it is.  

That’s the problem that’s faced when determining the distance to distant stars and galaxies. Just how far are they away?  The moon, and anything further away, we perceive as 2 dimensional. We can’t get any 3-dimensional cues and so we have no idea, just by looking, of how far away they are.  So how can we measure distance to the stars? 

One way, which works for the nearest stars, is parallax. The earth orbits the sun, and six months from now it will be about 300 million km away from where it is now. That gives a different viewpoint. The nearest stars, therefore, appear to move against the background of stars that are further away. We can therefore use a bit of simple trigonometry to work out the distance to the star. Indeed, one of the units of distance in astronomy is the parsec – one parsec being the distance over which the diameter of the earth’s orbit subtends a parallax angle of one arc-second.  Essentially, using parallax in this manner is like viewing the situation with two eyes – 300 million km apart.

Parallax, however, only works for our nearest stars, since the distances to our neighbours are so huge. To work out distances further away, there are other methods – such as looking at the intensity of Cephid Variable stars, and, for really long distances, the famous redshift. However, somewhat disappointingly, neither of these are exemplified by the Hobbiton movie set.

Time travel Marcus Wilson Aug 24

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On Monday evening this week I managed to do a bit of time travelling while driving back home. I was driving back through one of those heavy showers that have been marauding around the place recently, with windscreen wipers full pelt on a rather wet road. However, these showers don’t last for very long, and the rain soon began to ease. As it did so, I noticed the road was getting drier.  Then the rain stopped all together, and I was left driving on an absolutely dry road.

That’s not what usually happens. Usually, the more rain there is, the wetter the road gets. I’m sure you’ve worked out what was happening.  I was heading in the same direction as the shower, but going faster than it. So I had overtaken it, as it were, and emerged from the rain ahead of the shower. There was a dry road, because the shower hadn’t got there yet.  Sure enough, I got home in the dry but within a few minutes it was raining – the same shower that I’d just driven through.

So I was experiencing the events of the rain shower backwards, because I was travelling faster than it. In one sense it was time travel. I was seeing events happen in a different order from what someone stationary on the ground would have seen.

Of course, it wasn’t really time travel. My clock was still going forward, as was everyone else’s. Now, if I’d been travelling faster than light, things might have been a little different. Special relativity says that time slows down for an observer travelling quickly  (from the point of view of someone who isn’t).   As this traveller approaches the speed of light, special relativity says that the passing of time for him becomes very slow indeed. In fact, at the speed of light, time wouldn’t pass at all for him. That’s one of the reasons that photons, light ‘particles’, behave very oddly.

What about beyond the speed of light? Physics as we know it doesn’t let us go there, not even with those neutrinos at Gran Sasso. If that result had been true, our understanding of physics would have been shaken up quite severely. The possibility of really travelling backwards in time might then have become a reality.

[ For those who are more mathematically inclined, the rain shower's also an example of why the partial derivative is not the same as the full derivative.  The full derivative for the rate of change of road wetness with respect to time was negative here - the road was getting dryer as I went alogn, but the partial derivative of road wetness with respect to time at constant position was still positive.  ]

 

 

Equipment failure Marcus Wilson Aug 13

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In the last couple of weeks, I’ve been working with one of our technicians tracking down what has been going wrong with one of the experiments we get our third year physics students to do. It’s on Brownian Motion. Specifically, analyze the movement of small particles suspended in water by scattering of laser light. By studying the way in which the scattered light varies in intensity with time, we can work out the size of the particles in the suspension.

So says the theory. However, in practice the pattern of scattered light is nothing like what we’d expect in this situation. There was clearly something going wrong, but working out what hasn’t been straightforward.

In the end, we just went through piece by piece through all the equipment, and the interfaces between the equipment, checking each was doing what it should have been. In the end Stewart worked it out – we had a dodgy oscilloscope. It’s rather easy to trust your instrumentation, especially that you’ve paid a lot of money for, but it is worth remembering that sometimes it breaks, and, when it breaks, it might not do so in a manner that is obvious. A piece of equipment that spits out the dummy and refuses to do anything is rather less dangerous than one that, on the face of it, is doing its job, but actually is getting it wrong. In this case the consequences of the fault are hardly serious – we’ve just had an experiment that was clearly giving puzzling and unbelievable results. In fact, for the last couple of years, I haven’t had the students even attempt it, because I’ve known something’s been amiss with it. However, that’s not always the case.

There are similarities I think with those faster-than-light neutrinos that hit the headlines last year. It was a crazy result – hence the attention – but on the face of it the experimental results appeared to be real. But, very careful checking of the apparatus highlighted a couple of glitches with the equipment. It wasn’t doing exactly what it was supposed to be doing. The problem was small, but it was big enough to produce a sensational result.

Fortunately, science has ways of correcting itself, and in due course the problem was tracked down by some careful investigation. It’s interesting that this is a skill we often overlook in teaching our students. In an effort to illustrate the theory, we present them with experiments that actually work (or at least try to). We never (certainly not here) deliberately give students dodgy equipment and then teach them how to find out what’s going wrong. Given that it is a skill that any experimentalist needs to have, it should be one we teach. Something to try in the future with another class of guinea pigs.

The shortest distance between two points Marcus Wilson Jul 30

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I remember as a student being presented with the proof that the shortest distance between two points is a straight line (at least, on a 2 dimensional flat surface). Although it’s almost blatantly obvious, it can be formally proved through Calculus of Variations.

However, the quickest route between two points is not necessarily a straight line. Take the example of my walk from my office, on the 3rd floor of EF block at Waikato, to the lecture room G.3.33 on the third floor of G block. Normally it is a short (1 minute) stroll down the third floor corridor in a straight line – the walkway goes from EF building into F building into G building and G.3.33 is straight in front of me. (Those ‘in-the-know’ will realise I’ve over-simplified the situation – there are a couple of 22.5 degree bends in the corridor and a few steps to negotiate, but basically it is a straight line between the two.) But not now. There is building work going on in FG link block. This means that the straight-line route is unavailable. Instead, I have to go down three floors, out the door, walk about 100 metres through the rain around the back of F and G buildings to the G front door, then up three flights of stairs, and walk back through the corridor to G.3.33. Total time more like 6 minutes for a distance of about 50 metres.

Here’s another example:  You are a lifeguard on a beach, say 50 metres from the water, and you see someone in trouble in the sea, say 25 metres off shore and 50 metres along the shore from where you are.  Which direction do you run in? You don’t run straight towards them, because you know you can run faster than you can swim. Covering the shortest distance in total would mean that you are swimming a longer distance than necessary. It is better to run more along the shore, and enter the water at a point closer to the man in trouble. You cover a longer distance in total, but less of that distance is spent in the water where you are going slowly.

The problem of refraction of light is analogous to the lifesaver description above. The light travels the path that gives the shortest time between two points. Suppose a light ray starts at point A, in air, and ends at point B, in a block of glass. Now, we know that glass has a refractive index. This means that the speed of light in the glass is lower than that in air.  The light doesn’t travel in a straight line between A and B, but rather in two straight line segments – in a straight line  from point A to point C on the surface of the glass, then in a straight line from point C to point B.  Where is point C? It turns out that it’s such that the total time the ray takes from A to B is minimized. Clever or what? How does it ‘know’ that this is the shortest path?

Many problems in physics are similar minimization problems.

 

 

How to cheat aging Marcus Wilson May 04

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It’s ‘Wellness Focus’ week here, and there are all kinds of wonderful activities going on to promote health among the employees of the University of Waikato. I’ve just finished a REV class, which has finished me off for the whole afternoon, I think.  Yesterday, I had a free health check – where my blood pressure, cholesterol etc was measured, and the nurse went through a lifestyle questionnaire with me assessing my risk of heart problems (which turns out to be low). One question on the list is ‘what is your age?’. It obviously affects your risk of a heart attack, though the nurse said it’s one factor that you can’t do anything about.

Well, I thought, that depends on how pedantic you want to be about it. I’ll be travelling to Sydney very soon as part of my study leave, and that is going to slow down my aging by a few nanoseconds. That’s simply a consequence of special relativity. The important factor here is denoted by the Greek letter gamma by physicists, and is the reciprocal of the square root of [1 - (v^2/c^2)].  In this expression, v is your velocity, and c is the velocity of light, or 299,792,458 metres per second.  (The ^2 means ‘square’). It tells you how much time slows when you are travelling at this speed. 

What is gamma for a commercial jet aircraft? At 900 km/h, or 250 m/s, it comes to 1.00000000000035 . That means, when I’m on board the plane, for every 1 second of aging I do, people on the ground will age 1.00000000000035 seconds, that is and extra  3.5 times 10 to the power of minus 13 seconds. Over the course of a three hour flight that comes about an extra 4 times 10 to the power of minus 9 seconds, or 4 ns.   Not enough so you’d notice, but it’s measurable with atomic clocks.

One of the problems with teaching special relativity is that its effects nearly all lie outside the realm of everyday experience. They are only apparent when something travels very fast. However, they are extremely important in physics. That’s the motivation for this piece of software I was told about a couple of weeks ago from the Australian National University in Canberra. The software, which you can download for free, lets you fly a spaceship around a city-scape at close to light speed, and observe some of the effects that are occurring, or observe different clocks at speed to see the time dilation effect.  It’s well worth a play, but to get the most out of it you should follow through the student instructions which come with it. The research article that comes with it is worth a read too.

 

Mysterious power generation Marcus Wilson Apr 30

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One consequence of being a physicist is that you can’t go anywhere without seeing physics calculations that need doing. I’ve just been to our library hunting down books on the medical technique of transcranial magnetic stimulation (TMS), which was an interesting exercise in itself, since one textbook I found also has a chapter on homeopathy. Hmm. So how much do I trust its section on TMS then?

Anyway, our new, glorious library and student centre gets better every time I go in. Not only has a new cafe opened up inside, but there are now screens telling you just how eco-friendly the building is being right now, by displaying data on the building’s power consumption, solar power generation, water consumption and water capture, etc.

So, in the last month (which I assume means April) the building has used 182 792 kWh of power (that certainly makes my electricity bill look tiddly!) but has generated a cool 1 847 531 kWh from its solar panels. Now, I know April has been unusually sunny this year (shame the sunshine couldn’t have come in summer when it was supposed to) but there is something clearly wrong with this figure.

One metre squared of area, under full sun, gets about 1 kW of power on it. That means in about an hour it captures 1 kWh of energy. I don’t know how much of the building is covered in solar panel or other capture device, but I reckon the footprint of the building is about square with a side of 40 or 50 metres, so let’s say about 2000 m2 in roof area. So, if that were covered in solar panel, under full sun it would capture about 2000 kWh in one hour. In April there are 720 hours, so that gives us 1 440 000 kWh of energy.

But I’ve assumed that the panels are illuminated 24 hours a day! That’s clearly rubbish. So halve that, since half the time it’s night. In April, the sun isn’t anywhere near the zenith, so let’s halve that again. We are down to about 400 000 kWh. Then there are cloudy days (albeit not too many this month) which will take it down again, and still a large factor to apply because the power conversion from light to electricity or hot water isn’t 100% efficient. I reckon we might be down to a more reasonable estimate of 100 000 kWh.

So what does the 1 847 531 kWh represent?  It’s either a mistake, or I’m misinterpreting the display.

 

The most interesting “photo” I’ve ever taken Marcus Wilson Apr 18

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This week I’ve started my Study Leave with a short visit to the University of Otago in Dunedin. Today, amongst other things,  I had a quick tour of one of the atomic physics labs there. Recently, Mikkel Andersen’s group has managed to trap a single atom of rubidium. It’s quite a complicated process – you can read about it in their Nature Physics article here. The main ideas are to grab a small number of atoms, trap them by holding them in place with beams of light, cool them down (again with light), and then, using some rather clever methods involving resonance (with, you guessed it, more light), remove one atom at a time until only one is left. Just one, single, solitary atom. Wow.

How do you know for sure? Take a photo of it, silly. And yes, that means yet more light. With a single atom, you can’t take a conventional photograph, because the wavelength of visible light is around 500 nanometres compared to the atom size of around a nanometre. It’s impossible to focus that small. But what you can do is measure exactly how much light is being scattered by the atom, and from that know for sure that there is only one atom there.

I had a go at this experiment. Admittedly, it wasn’t desperately taxing on my abilities as a physicist – in fact all I had to do was push a button on a computer window and in half a second the machine did its thing, completely silently, and produced a nice looking photo with a splurge of light in the centre. And that light was being scattered by just a single atom.

So, you CAN see single atoms, after all. I have a picture to prove it. (If you want to see one, have a look on the group’s website here).

 Postscript. I’m staying with my sister-in-law and her family. When I got back to their house this afternoon, I showed the picture to my eight year old niece and asked her if she could guess what it was. She gazed at it for a few seconds, then said "An atom?"

 

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