Why Does Adding a Third Polarizing Filter Let Light Pass Through?

[James Jackson]

Do others have bits of 'simple' (or, indeed, complex, but the simple has a bigger 'wow' factor, I reckon) Physics that make you go 'wow', 'huh?' and 'I dunno!' all in one fell swoop?

I recall a very easy demo in an early QM lecture - take two polarising polarizing filters arranged at 90 degrees to each other (say, on an overhead projector so you can see it well). Lo and behold no light. That makes sense.

Then add another filter on top of those two at 45 degrees. Lo and behold - light. That's completely crazy really. The mathematics works (and is very simple) BUT there wasn't light, and then there is. As my lecturer said (to paraphrase) 'If anyone can explain that in physical terms they'll become rather famous'.

Simple, but a touch wierd.

 

[ZapperZ]

I have a couple off the top of my head.

The first one I've mentioned on here a while back. I was riding on a CTA train (that's Chicago Transit Authority for those who don't know) when there was a bunch of kids going somewhere, probably either to or from a birthday party, and a few of them had these helium filled mylar balloons. I notice this at first but it didn't register to me that it was rather odd. Then finally it hit me that there's something rather strange. Whenever the train accelerates forward (I was facing forward), the balloons appeared to lunch forward in the same direction that the train is moving. I thought that was rather odd since during the acceleration, everyone feels a push backwards. So why would the balloons lunge forward?

I think I was staring at the balloons for a long time, especially whenever the train stops and starts moving again. They did this forward lunch each time the train accelerates. I didn't notice that clear of a pattern when the train was decelerating because at that time, the CTA trains are notorious for not slowing down smoothly, so the balloons jerked around quite a bit during the slow down part.

Of course, now I know the explanation for such a thing.

The 2nd "huh" moment came only recently, probably about a year ago. We were trying to do a "novel" photoemission process by applying a large electric field on the surface of a photocathode. By doing that, we could lower the effective work function of the material. So, if we use light with energy LOWER than the work function, and then apply a large enough E-field on the cathode, at some point, the effective work function will be low enough that we'd start seeing photoelectrons being emitted.

Well, that didn't happen. What happened was, even BEFORE we applied an E-field, we were already getting photoelectrons, even when the photon energy was lower than the work function - clearly a violation of the Einstein's photoelectric effect model. So this was a "huh", "what the...", "what's going on"-type of moment.

Of course, we found out what's going on eventually. It was also fruitful since we got a PRL out of it. :)

Zz.

 

[rocketboy]

I know its simple simple physics, but flight makes me go "wow". If you've ever been gliding you'll know what I mean. When you are getting towed up by the tow plane, and watching it you just can't help saying to yourself "wow, that is amazing" while watching the tow plane ahead of you soar through the sky. It's really different when you're right behind the plane watching it fly than say on the ground looking up at it.

Similarily a rocket taking off does the same thing for me.

This probably isn't what you were looking for lol but I couldn't help but share my fascination with flight.

 

[James Jackson]

ZapperZ - what was going on there? Sounds interesting...

Edit: The photoemission, not the balloons :)

 

[Nereid]

Quite a long time ago ... the realisation that the energies recorded for cosmic rays (well, some of them) were soooooo many OOM above what the (then, and even now) most powerful (human built) particle accelerators were capable of. Some time later, ditto, re gammas (TeV and above).

There was also the time I realized that if the solar system were in the path of the gammas we observe from GRBs, at 'short range' (i.e. many light years), we'd all be toast - how much energy does it take to vapourise the Earth? Well, a GRB has that much to 'donate' at a distance of many ly!

 

[ZapperZ]

ZapperZ - what was going on there? Sounds interesting...

Edit: The photoemission, not the balloons :)

OK, the setup was like this:

Photocathode: Mg, work function 3.7 eV
Photon energy: 3.3 eV

What I didn't tell you was that our light source is a very intense laser, roughly 2 to 10 mJ per pulse that is 6 to 8 ps long. So in one pulse, we're shooting A LOT of photons at the cathode. It turns out that we had such high photon density per unit area that we were causing 2-photon photoemission. This is where the first photon causes a transition to a state still below the vacuum level. However, the decay back to the ground state isn't instantaneous. It has a life time of the order of fs or even longer. Since our light source is so intense, before this excited state could decay, it absorbs another photon and that causes it to escape the material.

Of course, the cross-section for this to occur is much lower than if we had used photons with energy larger than the work function (the traditional photoelectric effect), so the photoelectrons we were measuring were a lot smaller in number.

Zz.

 

[paulhunn]

The first "Wow" moment for me in physics was when i read Brian Greene's "The fabric of the cosmos" It was when he pointed out that light from a distant quasar can be split and focused by an interving galaxy to create an interference pattern, if an additional detector was to be switched on then the pattern would disapear because which-path information had been provided. What amazed me was that it seemed like the photon had to decide billions of years ago if it was going to go one way round the galaxy as a particle or both ways as a wave.

 

[HungryChemist]

First thing that comes to my mind was my college parking sticker. How this thin parking sticker would just stick to your car window for a very very long time without glue. After having thought about it and talked about it with my freinds, I was even more amazed.

Another one was this demonstration. Put a piece of paper close to your mouth while your mouth is directing straight to ground, and blow air hard against paper, paper will stay on the air all by it self. It was very non-intuitive!

 

[jtbell]

The two "wows" that come to my mind first are both in the theory area:

1. Learning about Maxwell's Equations in freshman physics (Halliday & Resnick) and seeing the symmetries among them (especially when you add hypothetical terms for "magnetic charge").

2. In special relativity, learning that momentum and energy transform the same way as position and time, that is, learning about four-vectors.

 

[inha]

"Simple" things like basic QM. Fractional QHE blew me away (I still don't understand it properly).

And the most latest: When the fluorescence setup I'm working with just decided overnight to not work properly anymore. But the wow was more like "wow, I really want to punch someone in the mouth".

 

[zanazzi78]

The first WOW i had was when reading a book there was a chapter on about superfluidity, (i`d never heard of it before then) the book stated that if you stirred a cup of coffee it will eventually come to a stop due to viscous forces, when tried with helium cooled below 2.17K the spinning will never stop! WOW but the book continued ... if left entirely to it`s own devises, the fluid may crawl up the side of the bowl and over the top! That blew me away and to be honset still does!

The second, was just over a year ago, i was looking around the department of some prospective Uni`s in one there was a lady doing research into Dia-magnetism basiclly she`s trying to make a perfect magnet at room temp. She had a toy she bought over the internet to demo how hard it was to levatate an object using magnets, then she took one small magnet poured some liquid nitrogen over it and balanced a small piece of metal ontop of it, it just sat there spinning slowly, hovering in mid air over this one small magnet, I thought that was sooooooooooo cool.

 

[LENIN]

The first thing that really maide me WOW was when we were calculating how much mass does the sun lose every second due to fussion in the 11th grade (I don't remeber the exact nuber but it was huge). The second thing came a few minutes later when we disscused haw long it would take for the sun to lose all its mass this way.

 

[darkar]

Is nuclear bomb counts?

I would say ouch, the power of it is so devastating and it killed so many T.T

 

[darkar]

Well, that was the abuse of use of science anyway.

There are also a lot of things that make me WOW. For example, when I first studied the gravity and found that what goes up will come down. Then the use of electromagnets to levitate objects...

It seems to me that everytime I learn a new topic, it always give me a feel of WOW!

 

[DaveC426913]

Do others have bits of 'simple' (or, indeed, complex, but the simple has a bigger 'wow' factor, I reckon) Physics that make you go 'wow', 'huh?' and 'I dunno!' all in one fell swoop?

I recall a very easy demo in an early QM lecture - take two polarising polarizing filters arranged at 90 degrees to each other (say, on an overhead projector so you can see it well). Lo and behold no light. That makes sense.

Then add another filter on top of those two at 45 degrees. Lo and behold - light. That's completely crazy really. The mathematics works (and is very simple) BUT there wasn't light, and then there is. As my lecturer said (to paraphrase) 'If anyone can explain that in physical terms they'll become rather famous'.

Simple, but a touch wierd.

YES! This one fascinated me as well! It didn't hurt that, at the time, I was working in a photo shop, where I could do the experiment for myself. It works!

Of course, the effect is completely lost on my co-workers. Stupidheads.

 

[ZapperZ]

YES! This one fascinated me as well! It didn't hurt that, at the time, I was working in a photo shop, where I could do the experiment for myself. It works!

Of course, the effect is completely lost on my co-workers. Stupidheads.

OK, since you two are fascinated by optics, let me run this by you and see if you think this is another "wow" moment.

You have a standard double slit set with a monochromatic light source. If you put a screen at roughly ANY distance after the slit, you see the infamous double slit interference pattern, ya? So far so good.

Now, what if you put a typical, standard, convex thin lens after the slit? If you place it right (i.e. according to the thin lens equation), you would actually get an IMAGE of the double slit on the screen! Big deal?

Well, think about it. Since the lens is after the double slit, what it is "receiving" is the interference pattern (put your screen there and you'll see this). Yet, what the lens put out is not an image of the interference pattern, but rather tha image of the double slit itself!

So what is the lens doing? <evil grin>

Zz.

 

[zanazzi78]

Yet, what the lens put out is not an image of the interference pattern, but rather tha image of the double slit itself!

So what is the lens doing? <evil grin>

Zz.

Ok i`m tentativly going to have a go at answering this, just please don`t laught if i`m way off the mark :uhh:

A. The lense never recievce an interferance pattern. The action of placeing the lenses in the experiment changes the nature of the experiment, instead of the experiment 'looking' for interferance patters of monocromatic light, it`s now looking to see what comes throught one of the slits. There fore defining the light to be a photon and not a wave. the object that the lense is using is therefore not an interferance pattern but the photons coming throught he double slit so the image it produces is of the double slits.

 

[robousy]

Learning that the charge on the electron is NOT the ACTUAL charge - but the 'screened' charge due to the surrounding virtual particle field.

 

[zanazzi78]

Learning that the charge on the electron is NOT the ACTUAL charge - but the 'screened' charge due to the surrounding virtual particle field.

what I`m lost ...

edit: Oh hang on It get it "things that make you go WOW" i got confused sorry

 

[Antiphon]

THERE'S A PROBLEM WITH THE OP!

That situation is correctly described by classical physics. No wow
is needed. Not even quantum mechanics.

The 45-degree intermediate polarizer consists of (essentialy) closely
spaced wires at a 45-degree tilt to the incoming polarization.

The currrents which are induced in the polarizer re-radiate energy
at 45 degrees and some of this gets through the second polarizer.

If you really want to blow your professor's mind, tell him that there's
nothing but Maxwell's equations in the explanation.

 

[James Jackson]

No, it's not an 'intermediate polarizer'. This is the principle that LCDs work with, and that is conceivable with the explanation you give (if it is rather lacking in 'true' Physics).

The situation I've described is that you have two linear polarizers at 90 degrees to each other, so you have no light coming through. None at all. You look at it, it's black. Then, you add the polarizer at 45 degrees ON TOP of these two, not in the middle, but at the top. You then get light output.

To the others: Yep, these things are all cool.

ZapperZ: That's an interesting process indeed. Hadn't heard of that before - I wonder if a couple of friends of mine (involved in various forms of high-energy laser spectroscopy) come across similar effects. RE: The optics thing, that's got me thinking. Damn, I hate optics...

 

[Antiphon]

The situation I've described is that you have two linear polarizers at 90 degrees to each other, so you have no light coming through. None at all. You look at it, it's black. Then, you add the polarizer at 45 degrees ON TOP of these two, not in the middle, but at the top. You then get light output.

This doesn't make sense. Can you explain what you mean by "on top of"
and not in between? Do you mean "in front of"?

I'll assume you mean "in front of."

If you are saying that you have 3 linear polarizers stacked in order in the following orientations:

1- 45
2- 0
3- 90

and light still gets though, I don't believe it. If the polarizers are separated
by air gaps, I know nothing will get through. If you are stacking them
in intimate contact, maybe there some leakage hapening somehow through
the polarizer substrate.

Could you please describe in brief detail the position, orientation and
materials used in this polaraizer arrangement?

I believe what you are describing is impossible as I currently understand the laboratory setup,
and I believe you saw what you say you saw so I must not understand the setup.

 

[cscott]

I believe what you are describing is impossible as I currently understand the laboratory setup,
and I believe you saw what you say you saw so I must not understand the setup.

I believe what the OP stated is described in Gribbin's "In Search of Schrodinger's Cat," in case you had the book handy :tongue:

 

[James Jackson]

Did you see the title of the post (Things that make you go 'wow').

Try it youself. Get three linear polarizers and an overhead projector.

Put one on - light intensity drops by a certain amount (1/sqrt(2) or something like that) as expected. Now put other on top at 90 degrees to the first. As expected, no light. Then put another at 45 degrees to those two on top of the pile. Lo and behold, light. It's not due to 'leakage', it's quantum, innit?

'If the polarizers are separated by air gaps, I know nothing will get through.'

Really? Try it.

 

[ZapperZ]

This demo is almost a standard in optics classes. I know I've done it. However, in case there's a possibility that the demo isn't clear, here it is again.

2 linear polarizers, one after the other, with polarization direction oriented 90 degress to each other. Using an unpolarized light source, if you like, you will see nothing getting through after both polarizers. However, if you put another polarizer IN BETWEEN the two, oriented let's say 45 degress from the first and second, then you will now get light passing through.

Zz.

 

[Ivan Seeking]

This demo is almost a standard in optics classes. I know I've done it. However, in case there's a possibility that the demo isn't clear, here it is again.

2 linear polarizers, one after the other, with polarization direction oriented 90 degress to each other. Using an unpolarized light source, if you like, you will see nothing getting through after both polarizers. However, if you put another polarizer IN BETWEEN the two, oriented let's say 45 degress from the first and second, then you will now get light passing through.

Zz.

That is a very cool demo; for newbies esp. Another similar example is seen when one opens a second slit in a double slit experiment, which causes an area on the screen lit by a single slit to go dark.

One of the first big wows hit me the first time I grabbed a 250,000 volt Tesla coil.

 

[James Jackson]

ZapperZ - No no, you can put the 45 degree polarizer on top of the other two. Try it...

 

[ZapperZ]

ZapperZ - No no, you can put the 45 degree polarizer on top of the other two. Try it...

Oooh.. now THAT I did not know about. I may have read your initial posting too quickly and immediately jumped onto the familiar demo.

Zz.

 

[brewnog]

My wows are more engineery, I'm afraid, and they're not exactly puzzling, but made my jaw hit the floor the first time I saw them.

Seeing a loading test done on a wing of an Airbus was pretty impressive. They strap the whole fuselage down to the floor, and basically pull the wing up from its tip (and several points along its section) using big pulleys from an overhead rig, to test it to destruction. The wing gets to something like 45 degrees above horizontal before it completely shears from the fuselage.

And playing with a supersonic wind tunnel is always fun. You can actually see the shockwaves forming around the aerofoil section. Bloody noisy though.

Finally, watching welding robots work in a car factory is absolutely mesmerising. The speed with which those little critters can swing over to a car body, reach inside the filler nozzle, turn round through 180 degrees, do a load of spot welds, and then remove itself before moving onto the next car is absolutely staggering.



Edit: Ooh, and one more! Calculating the angle of twist which occurs over an oil rig twist-drill can give some surprising results!

 

[DarkEternal]

hmm...i think maybe YOU had better go try it, james jackson. i just did, and as far as i can tell, it just confirmed what i originally thought: the 45 degree polarizer has to go BETWEEN the other two for light to transmit.

 

[RandallB]

ZapperZ - No no, you can put the 45 degree polarizer on top of the other two. Try it...

I agree with DarkEternal
I had to put the 45 in-between no matter how close.
Let us know what you see on a retest, JJ.

Although, I'd credit the standard test as Zz described it as worthy of a WOW considering it still works even if only one photon at a time is sent through. Classical can’t explain that; and QM only has formulas to predict it on average, but not explain the individual actions.

 

[Nenad]

A first wow for me was the single slit diffraction pattern and how it was explained.