Now Brady, here comes the bit that really gives you a headache...
I'm going to show you how light can be polarized by sheets of Polaroid.
This is a sheet of , it's polyvinyl-alcohol with iodine in it.
In here are long molecules which are stretched out
and conduct along this direction but they don't conduct that way.
That's what your Polaroid sunglasses are made of.
It reduces the glare of the road.
I put this sheet on top of this overhead projector
and I put another sheet on top polarized the same way
and they absorb a bit of light.
But if I turn this top one round, so that it is perpendicular,
you can see you can kill off most of the light.
So there is some turning backwards, all the way round,
and now I'm going back again and you see it disappears.
This is an off-cut, a bit chopped off from the bottom of this,
and I can put it on top of these two, and you can't see it at all.
I can put it underneath these two and you still can't see it.
But if I now slide it in between... sliding it in between,
you see you can see it!
If I turn it around, it disappears,
if I turn it around 45 degrees you can see something quite strong,
then turn it around a bit more and it disappears.
It's magic, now you see it... now you don't.
Now you see it... now you don't.
- Amazing, what's going on?
- Well, I find this absolutely fascinating, I didn't believe this was going to work(!)
But I'll show you set of equipment over here,
which is some microwaves.
This is a microwave generator and it generates waves in one direction.
And this is a receiver which will receive them.
This wave has a wavelength of three centimeters.
Now, three centimeters is about this long.
The light wave is 60,000 times smaller.
If I set up traveling waves, coming out of here and ending up here,
then it would be a bit simpler.
So I'll detect this by turning this on, getting a very nice signal.
Now this is my sheet of "Polaroid"
it's just long conducting wires.
The same as in the sheets of Polaroid, but you couldn't see them.
Now I put it in this way round,
and you don't get a signal,
and I turn it round, and get a huge signal.
So, there it is, nothing gets through
and I turn it around slowly, and you see it reappear.
And when it's perpendicular, I hit a really big signal.
What's happening here, is that this sends out waves oscillating in this direction.
and as the electric field hits this conduction wires
it induces a current in here, and because of the resistance
of the wire the electromagnetic wave gets to be absorbed and heat the wire.
So it gets totally absorbed by the wire.
If I turn it around this way,
now the electric field causes a little bit of current,
but then it hits the other side of the wire, and it doesn't dissipate,
it gets stored as a capacitant.
So this does not absorb nearly as much energy,
so I get a large signal. And then I turn it around, there I get virtually nothing.
Now I want to turn this around, to show you they're polarized oppositely.
So if I have that vertically, this is giving off waves oscillating vertically,
this is detecting only waves which are going horizontally.
Just like those sheets of Polaroid there.
So I have a wave that is oscillating this way,
and this one is not tuned to detect it at all, so I get no signal.
Now I put the sheet of Polaroid in, and nothings there,
if I turn it 45 degrees, I get a huge signal going through.
If I turn it perpendicular I get nothing.
So I start off vertical, there's nothing,
I turn it round, get a lovely big signal at about 45 degrees.
And when I turn further round, it decays away to zero.
So something is happening if I put this in at 45 degrees.
I'm able to get waves oscillating this way,
to go through here, and be detected by something
which is detecting waves that way.
I'm changing this signal which is oscillating this way
to one that's oscillating that way, otherwise nothing would go through.
- How have you done that? - Well I haven't done it, it's this sheet of Polaroid.
If I imagine this is a wave with a vertical polarization,
I can think of it as partly this side of the triangle and part that one.
So as I put it in at this angle, if I think of it as vertically polarized,
it's partly this and partly that, so as I put in this in there,
the bit which is running this way is killed off,
and the bit which is running that way is allowed to continue.
So I have a state which is oscillating this way now,
this thing has managed to turn the plane of polarization of the wave,
from vertical to this direction.
Now that this waves travels along this way,
part of it is horizontal and the horizontal part goes in.
I have a wave, I measure it, it changes the state
from being vertically polarized to one polarized in this direction.
Once I've done that I can detect it here.
That's the interpretation of this whole experiment.
And that's the classical interpretation in terms of electromagnetic waves.
Now Brady, this is the bit that really is going to do your head ache,
which I'm going to enjoy!
Out of this apparatus comes one photon,
it is a photon oscillating this way.
The chances of this photon going through here, are 50:50.
It can either be this way or this way and
if I do it at 45 degrees, they're equally probable.
Then you get to this detector,
and now it's coming out with this polarization,
the chances of it going this way are 50:50.
So I get only a quarter of the signal going through
and that's why the signal has gone down.
Notice now, as this photons comes through,
I have to talk about probabilities. It is not deterministic.
And if you go through the mathematics,
you'll find that these states,
which I would like to represent using a triangle like this,
and you put the symbol in, that's a triangle,
that state changes from one to the other.
In here, in between, I have a mixture of two different states,
and it is the mixture of two states that allows me to interpret this experiment.
So it's much harder for poor old Brady to understand,
because somehow quantum mechanics is biting him on the backside
and he really doesn't like me to talk about mathematics.
But without it, without splitting this into two different states
added together I have no explanation for what's going on.
Get your head around that Brady! - I'll just edit that out. How do sunglasses work then?
- Sunglasses work, well I haven't got them,
but I used to have clip-on Polaroids,
somehow if you're looking at light which is reflected off the road,
the light that's polarized in this direction is taken away,
so you find that you have transverse polarization only,
if you get rid of that, you reduce the glare of the road.
You don't get a nasty reflection on the road.
Also if you look at the surface of a river of pond,
it'll get rid of the glare and you can see into the pond.
The other reason they work is that they seem to attract the girls,
if you're cool, unlike me.
transcription: merijn.vogel@gmail.com