I need your help teaching physics

[ΔxΔp≥ћ/2]

For my physics classes I have to create presentations on a subject in physics. I was feeling ambitious and chose QM for my physics 3 class. I must do a 5 minute presentation (am shooting for 15 minutes).

Now, I know I will have to surprise them with how reality is perceived in quantum mechanics, I will do this mostly by showing experiments and physical phenomena: Davisson-Germer, Double-slit, maybe Aspect, blackbody radiation, nucleosynthesis, Hawking radiation, half-life, electron in a box, the Compton effect and I will demonstrate the photo-electric effect and maybe demonstrate the emission spectrum of a gas.

I plan on dividing the presentation into sections : complementarity, the uncertainty principle and maybe the exclusion principle (I don't think time will allow).

I also have to create some kind of model. I will be demonstrating the photoelectric effect, but I would like to have another one too. Any ideas on that one? Is it possible to do something like the Young experiment and show the interference of light in the classroom?

That’s the presentation in a nutshell.

I know that you guys have explained quantum mechanics many times and some of you even teach quantum mechanics, so I'm hoping that you can help me in making the presentation as logical and easy to understand in the allotted time.

Remember: I want to cover the most quantum mechanics possible in a short period, without missing any major parts of the theory and without losing the class.

 

[blechman]

One really nice demo of QM is the following: take a polarizer (sun glasses) and shine light through it. Now take another polarizer and rotate it. You will see the light get dimmer until it is black (when the two polarizers are perpendicular to each other). So far, nothing but classical E&M.

Now insert a third polarizer in between them, and POOF! It is no longer black! By inserting the middle polarizer you have collapsed the wavefunction of the photons a different way and this allows light through the final polarizer. Very simple, yet very profound.

Treating QM in 15 minutes, that's quite an ambitious project! May I suggest that you try to focus on only a couple of the many beautiful experiments you mentioned. I know you want to do it all, but there's just no time, and you'd be better off doing one or two of them REALLY well rather than doing all of them terribly. That's my advice.

Good Luck! Let us know how it goes!

Let me also emphasize (as you seem to be doing) that you should STAY AWAY from the equations, and stick to demos - this is coming from a particle theorist! If you start writing equations down, you'll just lose your audience. But with a good demo, they'll be yours for life!

 

[ΔxΔp≥ћ/2]

Thanks for the help. I think I will do that experiment.

As for the presentation, I would probably be better off just doing complementarity and the uncertainty principle then (in that order).

As for all the experiments, I wasn't planning on doing most of them in depth. For example: it is theoretically impossible to prove that the sun shines unless we allow for tunnelling. (Hopefully with a better wording than that though).

I would have really liked to do like Feynman and Brian Greene and start with the double slit experiment, but it is so good at explaining QM (the important principles are all present) that I fear that it would be impossible to explain them one by one. Should I maybe finish with it to go through all the concepts again?

As for the math, as a general rule, I will have no time to go through the proofs (particularly that some of the math does not even have a proof). However I am considering the possibility of including ΔxΔp≥ћ/2. I know that it is a special case, but how special is it and is it easy to prove? We are also learning matrixes in math class, should I talk about this representation at all?

I would like to get into the history of QM too (it is a passion of mine), but only in passing.

There are some other things that I was planning on staying away from due to complexity or disagreement among scientists, but you guys might have a different opinion. Schrondinger's cat (multiple states) and the interpretations: Feynman(all possibilities cancel each other, could be a mess), Copenhagen(the moon is not there when you are not looking?!?), Bohm, Multiple Worlds(messiest) etc.

I have already read a few books on QM, are there any that I should absolutely read before I present? Any of them with nice pictures I can show?

As always, I really appreciate your help.

 

[blechman]

I would have really liked to do like Feynman and Brian Greene and start with the double slit experiment, but it is so good at explaining QM (the important principles are all present) that I fear that it would be impossible to explain them one by one. Should I maybe finish with it to go through all the concepts again?

the double-slit experiment is where it all begins! It should be very easy to construct for light (Young's experiment, pre-QM), and then you can explain that the exact same thing happens when you shine a neutron beam through the slits! So much for neutrons being "particles"! Unfortunately, it's a bit harder to actually do that experiment.

As for the math, as a general rule, I will have no time to go through the proofs (particularly that some of the math does not even have a proof). However I am considering the possibility of including ΔxΔp≥ћ/2. I know that it is a special case, but how special is it and is it easy to prove? We are also learning matrixes in math class, should I talk about this representation at all?

NO MATH! - you can "prove" the uncertainty principle with words: let's say you want to measure the position of a particle very precisely. How do you do it, physically? You have to shine light on it to "see" it and measure it. More precisely, you scatter photons off of it and watch the diffraction patterns that result. HOWEVER, by the principles of QM, some of the momentum of the light will be passed onto the particle you want to measure, screwing up your measurement of its momentum.

This "proof" should be enough for high-school students (and teachers!) The mathematical proof of the HUP is not difficult, but it involves some rather high-level mathematics. You can prove it explicitly by considering various specific examples (famous QM1 problem: calculate Dx*Dp for the physical problem at hand and show that it is always >= hbar/2) - but you don't want to go this route in a 15 minute presentation - no one will understand you.

BTW: the HUP is always true - it's quite general and mathematically precise (see the QM1 problem that I mentioned earlier - you can explicitly calculate Dx and Dp and show that the HUP holds).

And don't talk about "matrix mechanics" and "path integrals" - this is *much* too advanced for your audience. Skip that entirely. There's no physics there - just mathematical formalisms. If you *must* talk about something, stick to wave mechanics - it highlights the wave-particle duality that is hidden in the other formalisms. But if you want my advice, I wouldn't even mention it. I suppose you could say something about "wave-function collapse" and all that (see below).

I would like to get into the history of QM too (it is a passion of mine), but only in passing.

It's such a fascinating subject, but I worry that you just won't have the time. Remember the rule I said above: better to do fewer things REALLY well than to do too many things crapily!

There are some other things that I was planning on staying away from due to complexity or disagreement among scientists, but you guys might have a different opinion. Schrondinger's cat (multiple states) and the interpretations: Feynman(all possibilities cancel each other, could be a mess), Copenhagen(the moon is not there when you are not looking?!?), Bohm, Multiple Worlds(messiest) etc.

I *hate* Schrodinger's cat! I find that it always leads laypeople to make mistakes. Did you know that Schrodinger proposed his cat example because he thought QM was garbage, and he wanted to emphasize that it made ridiculous predictions? His luck, it became the standard example of how "cool" QM can be.

As I said above: stay away from Feynman's path integrals. This is a mathematical technicality, not "deep physics".

You can talk about wavefunction collapse for real (is the electron spin-up or spin-down?) and mention that there are various ways to "interpret" this (Copenhagen, many worlds, etc). But I would be careful here: there are many pitfalls, and you don't want to sound like you don't know what you're doing if this is for a class!

I have already read a few books on QM, are there any that I should absolutely read before I present? Any of them with nice pictures I can show?

The first book on QM I ever read was Nick Herbert's "Quantum Reality" (to be honest, it was so long ago, I don't remember if I thought it was a good book or not!). Gibbons's pop-science book, "In Search of Schrodinger's Cat" is also very good.

As always, I really appreciate your help.

I hope this does help. Let us know how it goes.

 

[ΔxΔp≥ћ/2]

I am in the process of finishing "In Search of Schrodinger's Cat" for the second time.

I am concerned that the students (and my uber-classical teacher) will think that I am wrong or will get the wrong picture by clinging on to a classical reality. How do I show that the uncertainty principle is not just from the awkwardness of experiment?

After further thought, the experiment with the Polaroid lenses bothers me too. I fear that all the class will get out of it is that we change the photon’s spin, instead of what they should. If I did choose to do it, could I just bust a bunch of regular sunglasses and run a laser pointer trough?

I am also scared that they favour either particles or waves (probably the former). I know how to explain the double-slit experiment in terms of particles, but how do you explain blackbody radiation and the photoelectric effect in terms of waves?

I have done a lot of reading and think I have a good grasp of the theory, I just want to get these things right.

Thank you so much blechman for taking the time to read and respond to my posts, your help is greatly appreciated.

 

[blechman]

I am in the process of finishing "In Search of Schrodinger's Cat" for the second time.

That's a good book!

I am concerned that the students (and my uber-classical teacher) will think that I am wrong or will get the wrong picture by clinging on to a classical reality. How do I show that the uncertainty principle is not just from the awkwardness of experiment?

But it isn't: it is that awkwardness of experiment that makes the HUP happen. The new feature in QM is that the awkwardness of the experiment is built in to the theory! That is: there is NO WAY to get around it, even in principle!

There is also a "classical" HUP that follows from wave mechanics (classical waves, this time) - if a wave is damped (which every physical wave is), there is a relationship:

\omega\Gamma\ge 1

where \omega is the frequency and \Gamma is the inverse lifetime. This follows from the equations for waves, and it is sometimes called the "Classical Uncertainty Principle". So the HUP also comes out of wave mechanics.

I say this to you, not suggesting that you should talk about it in your presentation, but just so you can see personally where these things come from (Heisenberg didn't "discover" the HUP, he just related it to "particles").

After further thought, the experiment with the Polaroid lenses bothers me too. I fear that all the class will get out of it is that we change the photon’s spin, instead of what they should. If I did choose to do it, could I just bust a bunch of regular sunglasses and run a laser pointer trough?

No to your first point: there is no way to explain this phenomena without QM and the collapse of the wavefunction. If you think of these as "classical" spins, you would still get zero transmission through the second (perpendicular) polarizer. Remember that a polarizer doesn't "change" the spins; rather, it blocks all the photons that have the "wrong" spin. So if it were classical, once the first polarizer (horrizontal, say) blocks the "vertical" photons, then they're gone, so there should be no transmission through the vertical polarizer whether the third polarizer is there or not. But with QM, the situation is different, since the polarizer at a 45 degree angle will re-introduce vertical photons due to QM, and that would never happen in a classical system.

As to how to do this experiment: see if you can get a polarizer from your teacher - they're *very* cheap. If not, you can pick it up at a science supply store or a local hardware store probably. If that fails, then (polarized) sunglass lenses will work.

Don't use a laser - that's coherent light, and may not have all the same effects you want. Just use a flashlight, or an incandescent bulb.

I am also scared that they favour either particles or waves (probably the former). I know how to explain the double-slit experiment in terms of particles, but how do you explain blackbody radiation and the photoelectric effect in terms of waves?

I'll get back to you on this last one. I"m off to dinner!

 

[nrqed]

I am in the process of finishing "In Search of Schrodinger's Cat" for the second time.

I am concerned that the students (and my uber-classical teacher) will think that I am wrong or will get the wrong picture by clinging on to a classical reality. How do I show that the uncertainty principle is not just from the awkwardness of experiment?

It's sad that a student would have to convince a teacher about something established for almost 100 years!

After further thought, the experiment with the Polaroid lenses bothers me too. I fear that all the class will get out of it is that we change the photon’s spin, instead of what they should. If I did choose to do it, could I just bust a bunch of regular sunglasses and run a laser pointer trough?

I am also scared that they favour either particles or waves (probably the former). I know how to explain the double-slit experiment in terms of particles, but how do you explain blackbody radiation and the photoelectric effect in terms of waves?

I have done a lot of reading and think I have a good grasp of the theory, I just want to get these things right.

Thank you so much blechman for taking the time to read and respond to my posts, your help is greatly appreciated.

I think you should really focus on a single concept and make it clear rather than pile up a bunch of stuff covered very quickly.

I think that the double slit experiment conveys all the weirdness of the particle-wave duality in a simple setup and can be explained with no maths. I would think that would be a good example to focus on. The wave-particle duality can be made explicit.

 

[blechman]

Once again, I would like to explicitly agree with what nrqed said. I would consider either the polarizer experiment or the double-slit experiment and give a great presentation on that. I think it would really be a hit.

You're not going to teach anyone QM in a 15 minute presentation!

 

[ΔxΔp≥ћ/2]

But it isn't: it is that awkwardness of experiment that makes the HUP happen. The new feature in QM is that the awkwardness of the experiment is built into the theory! That is: there is NO WAY to get around it, even in principle!

I'll get back to you on this one.

So... ...we use the wave function to establish probabilities of finding particles in a certain spot. Observation (interaction with particles real or virtual?) causes the wave function to collapse, we see a particle.

No to your first point: there is no way to explain this phenomena without QM and the collapse of the wavefunction. If you think of these as "classical" spins, you would still get zero transmission through the second (perpendicular) polarizer. Remember that a polarizer doesn't "change" the spins; rather, it blocks all the photons that have the "wrong" spin. So if it were classical, once the first polarizer (horrizontal, say) blocks the "vertical" photons, then they're gone, so there should be no transmission through the vertical polarizer whether the third polarizer is there or not. But with QM, the situation is different, since the polarizer at a 45 degree angle will re-introduce vertical photons due to QM, and that would never happen in a classical system.

I believe you, but how can we say that the photons are not just deflected (thinking of a classical spin) by the 45degree polarizer and re-deflected by the perpendicular one instead of reintroducing photons?

It's sad that a student would have to convince a teacher about something established for almost 100 years!

I think he is waiting for them (physicists) to agree...

By the way, my presentation is in January, so you will hear plenty from me between now and then. I am also doing a project on special relativity.
Thanks, you guys are great.

 

[blechman]

So... ...we use the wave function to establish probabilities of finding particles in a certain spot. Observation (interaction with particles real or virtual?) causes the wave function to collapse, we see a particle.

That's a little confusing to me. I would say, rather, that we make a "measurement" and this collapses the wavefunction, so that all future measurements yield the same result. What, precisely, this "measurement" thing is, is actually a deeply troubling and still-unsolved problem of QM.

I believe you, but how can we say that the photons are not just deflected (thinking of a classical spin) by the 45degree polarizer and re-deflected by the perpendicular one instead of reintroducing photons?

I'm not sure I understand you. Think of a polarizer as a (vertical, say) metal grate with a string going through it. Wiggle the string in the vertical direction (making vertically polarized waves) and they go through the grate, no trouble. Send horizontal waves through, they're blocked. Send a wave polarized at an angle: you get a (smaller) vertical wave coming out! So polarizers don't "deflect" anything, they BLOCK!

I am also doing a project on special relativity.
Thanks, you guys are great.

Have fun!

 

[ΔxΔp≥ћ/2]

I'm not sure I understand you. Think of a polarizer as a (vertical, say) metal grate with a string going through it. Wiggle the string in the vertical direction (making vertically polarized waves) and they go through the grate, no trouble. Send horizontal waves through, they're blocked. Send a wave polarized at an angle: you get a (smaller) vertical wave coming out! So polarizers don't "deflect" anything, they BLOCK!

That example helps a lot.

If I physically did the string experiment, it would wiggle horizontally though right (seeing as the grate doesn't block the string in the same way)?

I know you guys are really insisting on ONE concept, but do you really think it would be hard to explain both wave-particle duality and the uncertainty principle in 15 minutes? Seems to me like I have done more stuff in less time before (a good special relativity presentation in 10 minutes).

Thanks again.

 

[blechman]

If I physically did the string experiment, it would wiggle horizontally though right (seeing as the grate doesn't block the string in the same way)?

If the slits of the grate a vertical, then wiggling the string vertically would send the wave through, no blocking. If you wiggle the string horizontally, no wave would pass through the grate.

I know you guys are really insisting on ONE concept, but do you really think it would be hard to explain both wave-particle duality and the uncertainty principle in 15 minutes? Seems to me like I have done more stuff in less time before (a good special relativity presentation in 10 minutes).

Hey, HUP, if you really want to do two things, none of us will stop you! But just keep in mind that "less, but better" is always preferable to "more, but worse"! The other point: this is not a public lecture, this is a class, and you don't want to look bad in front of your teacher and his red pen! So my advice (and nrqed, I think, would agree with me) is to consolidate, choose one (alright, maybe two!) things that you can explain *very* well with no math, and stick strictly to that. Remember that it takes time to run experiments/demos, even when it all goes right! You don't want to be rushed.

 

[ΔxΔp≥ћ/2]

your teacher and his red pen!

... ...after reading these words HUP runs and jumps out a window...

...but comes back to finish his post.

If I had a vertical polarizer followed by a second one 45degrees to it, wouldn't all the light coming out of this experiment be polarized at 45degrees from the vertical? If not, shouldn't no light get trough a system of two polarizers if they are not both parallel? I hope you see where I am going with this.

 

[blechman]

If I had a vertical polarizer followed by a second one 45degrees to it, wouldn't all the light coming out of this experiment be polarized at 45degrees from the vertical? If not, shouldn't no light get trough a system of two polarizers if they are not both parallel? I hope you see where I am going with this.

OH, I think I see what you're saying now. Yes, you're right. You can rotate the wave polarization in this way (and, in fact, that's how you do it!).

But you can also interpret light as a particle (photon), and it has one of two polarizations (that is is, a number +1 or -1). These quantities never "talk" to each other (they are "orthogonal states"). Now send it through the first polarizer, so only the +1 photons come out. Now there are no more -1 photons left, so you should never see them again! And yet I can regenerate them as if by magic with the middle polarizer!

Mad at me? Maybe we should just say: To Hell with the particle description! Except I can do the exact same experiment with neutrons (I stay away from electrons since they're charged and that complicates things, but you can do electrons also if you want!). Now, SURELY these are particles , and yet they will reproduce the same funny result of this experiment.

 

[nrqed]

That example helps a lot.

If I physically did the string experiment, it would wiggle horizontally though right (seeing as the grate doesn't block the string in the same way)?

I know you guys are really insisting on ONE concept, but do you really think it would be hard to explain both wave-particle duality and the uncertainty principle in 15 minutes? Seems to me like I have done more stuff in less time before (a good special relativity presentation in 10 minutes).

Thanks again.

I have seen numerous students giving their first technical presentations 9typically it was a talk to present their undergraduate thesis) and the most common mistake they make is to try to cram too much stuff in a one hour presentation and to get too technical (they presnet too many equations too quickly). This is recipe for disaster.

You have to keep things simple and clear. Especially given that you are presenting ideas that are very counterintuitive, so hard to grasp the first time one hears them! Focus on one concept illustrated with one experiment. And then just make sure that you present the setup very clearly (don't rush! Take the time to make things very clear, even things that may seem obvious to you) and explain the implications of the experiment in details, emphasizing the weird part. Practice several times and present the talk to people you know before you do it in class (preferably people who know nothing about physics and who will tell you if there are some stuff you say that are not clear to you). You will know it's a godo talk if someone with no physics background at all "gets it" and comes out of the presentation feeling amazed at the wirdness of the quantum world.

 

[nrqed]

I'll get back to you on this one.

So... ...we use the wave function to establish probabilities of finding particles in a certain spot. Observation (interaction with particles real or virtual?) causes the wave function to collapse, we see a particle.


I believe you, but how can we say that the photons are not just deflected (thinking of a classical spin) by the 45degree polarizer and re-deflected by the perpendicular one instead of reintroducing photons?

It's because of possible confusion about what a polarizer actually does (one must be already familiar with them in the contact of classical optics before really understanding the implications for the particle aspect of light) that I personally prefer the double slit experiment when I want to introduce the wave-particle duality to neophytes.

First, the interference pattern through a double slit is easy to viusalize with classical waves (one can even show an interference pattern produced with actual water waves or one can shine a laser beam through a double slit and see the pattern right away). The fun begins when discussing shooting electrons through a double slit and seeing an interference pattern appear. This is truly amazing. What is the wave here? what is "waving"? Then the next step is to explain that the wave involved here is a probability wave (and one must explain why this is the correct interpretation).

Then the fun begins when one tries to "catch" the electron going through one hole .

I could easily fill 15 minutes with just that!

One trick: try to build the "suspense" by asking leading questions, wondering aloud about some issues, emphasizing what is weird, etc.

 

[ΔxΔp≥ћ/2]

...ok, I think I'll scrap the polarizer. I still don't fully understand the concept and I am very familliar with the double-slit experiment.

I will go through the double-slit experiment in depth and use other "real world" examples to reinforce my point. I still want to demonstrate the photo-electric effect when I talk about the wave-particle duality though (it's pretty darn cool). Or should I consider dropping that too?

 

[nrqed]

...ok, I think I'll scrap the polarizer. I still don't fully understand the concept and I am very familliar with the double-slit experiment.

I will go through the double-slit experiment in depth and use other "real world" examples to reinforce my point. I still want to demonstrate the photo-electric effect when I talk about the wave-particle duality though (it's pretty darn cool). Or should I consider dropping that too?

To explain the photo-electric effect clearly (all the implications about changing the potential, changing the intensity of the light, the frequency, etc) would require more than 15 minutes in itself and there is a lot of background to get through before getting to the point. Of course you can do what you want but I would suggest to drop it.

 

[ΔxΔp≥ћ/2]

To explain the photo-electric effect clearly (all the implications about changing the potential, changing the intensity of the light, the frequency, etc) would require more than 15 minutes in itself and there is a lot of background to get through before getting to the point. Of course you can do what you want but I would suggest to drop it.

Don't you think that I need an example of light as particles or is particles as waves the best part?

 

[blechman]

...ok, I think I'll scrap the polarizer. I still don't fully understand the concept and I am very familliar with the double-slit experiment.

Sure, get rid of my suggestion. Thanks for the support, nrqed!

Just kidding. If you are at all confused about it and you feel like you understand double slit better, then you're quite right to focus on that instead.

Don't you think that I need an example of light as particles or is particles as waves the best part?

Again, if you had more time, then it would be nice to do this. But you said you're interested in science history, so consider this: when Newton wrote his (other Magnum Opus) "Optiks", he thought that light was a particle! It wasn't until much later, when Young did his experiment, that people decided that Newton was wrong and that light is actually a wave. So from a historical point of view, the idea of light being a particle was much less of a shock than matter particles being a wave!

Maybe that convinces you to focus on the double-slit; maybe it doesn't.

 

[nrqed]

Sure, get rid of my suggestion. Thanks for the support, nrqed!

Just kidding. If you are at all confused about it and you feel like you understand double slit better, then you're quite right to focus on that instead.

Sorry!

It was just a suggestion I made because I thought that the details about polarization even for a classical wave might make the presentation more difficult for HUP. I really did not mean to be rude in any way. Especially not with you since I know that I will learn a lot from you if you keep posting here!

 

[nrqed]

Don't you think that I need an example of light as particles or is particles as waves the best part?

But with the double slit experiment you can show both! After discussing the situation with electrons, you may go back to light and reveal that when one decreases the intensity of light to very very low level, one realizes that it's actually little bundles of energy that hit the screen: the photons! So that the actual ineterference pattern one sees with ordinary light appears this way only because a huge number of photons are sent every second and we don't notice the particle aspect of light unless we reduce the intensity to very low levels. So you get both particles as waves and light as particles with the same setup.

 

[ΔxΔp≥ћ/2]

The polarizer experiment is really cool though. I might revive it for my presentation in my EM class next semester.

Again, if you had more time, then it would be nice to do this. But you said you're interested in science history, so consider this: when Newton wrote his (other Magnum Opus) "Optiks", he thought that light was a particle! It wasn't until much later, when Young did his experiment, that people decided that Newton was wrong and that light is actually a wave. So from a historical point of view, the idea of light being a particle was much less of a shock than matter particles being a wave!

I am well aware of this, no math in quantum history :). I was intending on talking about that for a while, but it will probably have to be cut... ...and the last thing I want is my physics professor to think that Newton had anything to do with quantum theory.

 

[blechman]

It was just a suggestion I made because I thought that the details about polarization even for a classical wave might make the presentation more difficult for HUP. I really did not mean to be rude in any way. Especially not with you since I know that I will learn a lot from you if you keep posting here!

No hard feelings! I was just being silly.

 

[nrqed]

No hard feelings! I was just being silly.

I am very glad to hear that

 

[reilly]

Been there, done that. Almost everyone who gives a talk, until they are very experienced, tries to pack in way more than is possible. It would take more than an hour to cover all the things you mention in your initial post, probably at least three or four hours.

I'd suggest just one example for your entire talk. I'd choose Davisson-Germer. Quantum Theory is based on empirical data; atomic spectra, spin, radioactive decay, ...DG, for all practical purposes, directly shows that electrons show wave-like patterns when sent through crystals. That was a show-stopper. Some images would be good. That's already five minutes. Next, you might summarize QM's, development -- Schrodinger EQ; & probability interpretation. You would be pretty much done at that point. But, it's an excellent idea to "tell 'em what you told 'em" -- part of the Golden Rule of public speaking.

I'd advise against the Uncertainty Principle. You first need the probability/statidsticasl structure of QM, and you need the idea of a standard deviation or variance, by any name you want. Even 1/2 hour would be pushing it for Heisenberg's magic.

Keep it short and keep it simple. Practice in front of a mirror. If you use slides, use the rule of thumb that seven items on a page is the upper limit of people's ability to absorb the information.

Good luck,
Reilly Atkinson

 

[robousy]

time passes VERY quickly when giving a talk.

DO NOT BE TOO AMBITIOUS!

I can't overstate that. I've been lecturing for a few years now, and I can tell you that quality is far better than quantity.

If you aim to explain too many concepts in a short period of time you will lose the audience...and worse still...you will become AWARE of the audience becoming lost and it will lose you. very bad. you will not impress anyone. Clarity is better than a Blitzkrieg.

As has been stated earlier in this thread, aim for one or two ideas and present them well.

The polarizer expt. as the demonstration, and the photoelectric effect/ interference as an example of wave particle duality should suffice.

:)

 

[ΔxΔp≥ћ/2]

So... I've cut more stuff.

My teacher absolutely wants us to solve a problem for in our presentations, so I was hoping to use ΔxΔp≥ћ/2 to represent this nifty situation that I have decided to demonstrate:
http://www.youtube.com/watch?v=KT7xJ0tjB4A&feature=related

Problem is, going through wikipedia, Dirac's The Principles of Quantum Mechanics and Shankar's Principles of Quantum Mechanics (all rapidly). Gives me the impression that it won't be so easy to compute. How do I use this equation? If it is too hard, what equation could I use?

But it isn't: it is that awkwardness of experiment that makes the HUP happen. The new feature in QM is that the awkwardness of the experiment is built into the theory! That is: there is NO WAY to get around it, even in principle!

Seeing that the experiment in the above video does not disturb the photons... ...?

 

[Xeinstein]

Do you need help teaching physics or do you need help learning physics?

 

[blechman]

So, HUP, what precisely is the problem you intend to solve? There are several in that movie clip. Put it in the form of a question for us. Be exact.

 

[David_Harkin]

I had to do a similar thing in my physics class only i had to talk about bucky balls for 5 minutes. I just talked about carbon nano tubes and put a large spinning picture up of one and the teacher was impressed. The secret is to distract them with nice pictures and make up the rest, no one will argue with you ;)...oj

 

[ΔxΔp≥ћ/2]

Do you need help teaching physics or do you need help learning physics?

Well, a bit of both. As much as I am currently eating up physics books, I recognise my current inability to cope with university mathematics (I will start to address it). Luckily, I think I have also realized that the mathematics are paramount to the proper comprehension of quantum theory. I am asking for the help of people much more qualified than myself to assure as proper a representation of the theory as possible.

As far as "teaching" goes, the title I chose for the thread may be misleading. I am not a physics teacher, as stated in an earlier post, I am a high school student who will be presenting a project on quantum mechanics in my senior physics class. I know this is extremely ambitious. I am hoping to give the other students an enriching experience, presenting quantum mechanics to laypeople (and my teacher who probably has no idea what it is).

I thought it was important to start this thread to get opinions from physicists as to how I should present quantum mechanics, but it is also a lot easier on everybody than starting a new thread every time I have a little question.

So, HUP, what precisely is the problem you intend to solve? There are several in that movie clip. Put it in the form of a question for us. Be exact.

Firstly, I will describe the experiment in the video for those who would rather not watch (IMO it is worth your time though)

A laser beam is passed through a narrow slit. We can observe a red dot (from the laser) on a screen. We then progressively make the slit narrower and narrower and as we do so, the dot on the screen gets skinnier and skinnier. After a certain point, the slit becomes very small, so small in fact that the Heisenberg uncertainty relation comes into play. As we continue to make the slit narrower, we begin to know with increasing certainty the position of the photons passing through the slit. Therefore, the momentum of the photons becomes known with less certainty. What once was a narrow sliver of a dot on the screen now becomes wider and wider. COOL!

I thought this experiment was so darn cool that I had to replicate it, for my own fun and also for my presentation. I went shopping today and got all kinds of junk. I tried all kinds of stuff, but I do not get this counter-intuitive effect that silences nay-sayers. I tried a few things, but my main idea was to use a small wrench as the slit, it pretty much closes (looks closed to me, but a very small amount of light gets through. I also tried a vice and simply taking two pieces of metal and pushing them together, for the same function. I was using a small almost laser-like LED light (laser pointers are really, really expensive to do such a cheap experiment 30.00$).

Its kind of sad really that I do not know the equation that is my user name. As far as an exact problem, I do not have one. If I can make the above experiment work, I would love to prove mathematically that it works. blechman, I am assuming that when you ask me to describe precisely what the problem is, you are referring to numerical values. Nice round numbers would be nice, just to get the point across not to be all that accurate. Heck, if I used variables, I would be happy. I would like to be able to define "uncertainty" mathematically. ex. if you half the width of the slit, the momentum does what?

My teacher also demands that we offer up a math problem and solve it.

I am trying to know my material inside out, because I have heard that the teacher tends to ask very difficult/irrelevant questions.

For example: I have a Power Point slide after the De Broglie slide and before the Davisson-Germer slide about Bragg's law. It is a really quick slide and it's only point is to say: "At this time we had a tested law that explained how X-rays acted in a crystal, it is quite similar to the double-slit experiment." I think this is a logical thing to put there, but I am preparing for him to ask me the details about Bragg's law (like how it works mathematically), so I ether have to learn all this stuff (in this case not too hard) or get rid of this nice transitory slide.

He could also very well ask how to compute a wavefunction/the Schrodinger equation... :) (I'll try to be prepared for some of those)
or very precisely how the experiments are done/how a measurement is made (detector won't do; phosphor screen is better, hitting it with a photon won't do either)

So I make sure that I know as much as possible about every slide or I remove them if answering some pretty elaborate/irrelevant questions could be a doosy.

My Power Point is moving along nicely, I hope it is just about done. I am going to list what I currently have, along with my opinion of the slide, if any of you feel like leaving feedback. Nothing below is necessarily the exact wording or even all the wording, in fact I will be presenting in French. These are also not the titles of the slides.

1.title page
2.cute little introduction page (have not do it yet)
3.Newton though light was a particle (very short, might delete it)
4.Young's experiment (will demonstrate)
5.Interference (nice pictures, I hope to add a little math to this one)
6.Explaining the result of Young's experiment had it been particles (proving light is a wave)

7.Black body radiation (to solve a problem Max Plank had to put energy as discrete packets)
8.Planck's constant (don't really know what to put here)
9.The photo-electric effect (Einstein used some of Planck's work to explain an existing phenomenon, light has particle qualities)

I feel that slides 7,8,9 are essential to just briefly talk about the quantification of energy(this is quantum mechanics after all), the particle nature of light (and to show people to Einstein’s contribution to QM) and Planck's constant as a universal constant and quite important in quantum mechanics. This is the area I feel maybe the least comfortable about, because I think I will got bogged down with questions that are relevant to quantum mechanics, but off my course. (How does the photoelectric effect work, what is a black body, and what equation did Planck use...)

10.De Broglie hypothesis (What if particles acted like waves? I want to over-simplify this one and use E=mc^2 to explain his idea)
11.Bragg's law (just saying enough to set up the next slide)
12.Davisson-Germer experiment (particles do have wave properties)
13.Showing the observed interference of the Davisson-Germer experiment
14.C60 the largest molecule diffracted so-far (Is this still the case? Could I say interfered?)
15.The observed effects when the Davisson-Germer experiment is done one particle at a time
16.The observed effects when we try to see what slit a particle goes through
17.Here I would like to do a slide stating that all possible outcomes happen and interfere with themselves until an observation is made. (This is pretty deep though and might warrant a question that I can not even imagine)
18.Here I would like to talk about measurement in quantum mechanics (just before the uncertainty principle to keep them separate)
19.The uncertainty principle/Werner Heisenberg (stating the principle, maybe talking about what a particle in a box does when we make the box smaller and smaller and mentioning that Heisenberg helped develop matrix mechanics)
20.Erwin Schrodinger (show his equation, say that he computed quantum mechanics with a wave equation, making sure that the students know that it is not a classical wave)
21.P.A.M. Dirac (combined Heisenberg and Schrodinger into a better theoretical framework, show a bra-ket)
22.A brief talk about probability waves (this is a time where I want to re-emphasise that all possible outcomes happen and interfere with each-other until the system is observed. I also want to talk about the probabilistic nature of quantum mechanics, that probabilities exist not because we do not know enough, but that a particle can be in many states at the same time. I also want to re-emphasise that we are not talking about classical particles and waves, but things that really are neither)
23.A picture with all the probability clouds for atoms
24.Quantum tunnelling
25.Quantum tunnelling
26.Stellar Nucleosynthesis (as a quantum tunnelling phenomenon)
27.Alpha radiation (as well as half-life, re-emphasising the probabilistic nature)
28.Conclusion

I had to do a similar thing in my physics class only i had to talk about bucky balls for 5 minutes. I just talked about carbon nano tubes and put a large spinning picture up of one and the teacher was impressed. The secret is to distract them with nice pictures and make up the rest, no one will argue with you ;)...oj

Nice pictures make a big difference. I agree with you, I seek out visual aids but also really cool pictures.

 

[ΔxΔp≥ћ/2]

I don't wish to be annoying and I know that my last post was too long, but any help would be greatly appreciated. Since the deadline is coming up, this is a bump.

Any thoughts at all on the last post?

 

[blechman]

sorry, HUP. I've been travelling. I promise to get you a more useful response tomorrow.

one quick point: i thought you only had 5-15 minutes. you can NOT expect to get through 28 slides in that kind of time - I can barely do that in a one HOUR talk! *And* you're talking about a teacher known to ask lots of questions...

I can tell you what my first bit of advice is going to be: GET RID OF TWO THIRDS (2/3=0.67) of your slides; I don't care which ones! :

 

[David_Harkin]

sorry, HUP. I've been travelling. I promise to get you a more useful response tomorrow.

one quick point: i thought you only had 5-15 minutes. you can NOT expect to get through 28 slides in that kind of time - I can barely do that in a one HOUR talk! *And* you're talking about a teacher known to ask lots of questions...

I can tell you what my first bit of advice is going to be: GET RID OF TWO THIRDS (2/3=0.67) of your slides; I don't care which ones! :

Listen to blechman, if you do have fifteen minutes at maximum your going to have about 30 seconds a slide, do you think you can explain the De Broglie hypothesis in that time or quantum tunneling in a minute. My advice is keep it simple and keep the audiences attention, there is no point explaining a complex idea to someone that does not understand the basic principles behind it, teach them these. I would try to stick to things like the photoelectric effect and explain how in this situation the light energy is arrives or is observed arriving in discrete "packets" of energy. Maybe near the end of the presentation have one or two slides incorporating more advanced ideas but only briefly mention the outline of what these are and their place in modern physics.

 

[ΔxΔp≥ћ/2]

slides 1-3,9-11,13,14,20,21,23,25-27 (wich is exacly half) are all about 10-15 seconds long
The rest is marginally longer (30seconds with very few slides around a minute)
I could easily cut about 8 slides: 3, 8, (7 or 9), 11, 18, 20, 21, 27 this is about two minutes max. This could however dilute some of the content and add more responsability to other slides.
Maybe I am just bad at evaluating time.

 

[ΔxΔp≥ћ/2]

Listen to blechman, if you do have fifteen minutes at maximum your going to have about 30 seconds a slide, do you think you can explain the De Broglie hypothesis in that time or quantum tunneling in a minute. My advice is keep it simple and keep the audiences attention, there is no point explaining a complex idea to someone that does not understand the basic principles behind it, teach them these. I would try to stick to things like the photoelectric effect and explain how in this situation the light energy is arrives or is observed arriving in discrete "packets" of energy. Maybe near the end of the presentation have one or two slides incorporating more advanced ideas but only briefly mention the outline of what these are and their place in modern physics.

The De Broglie hypothesis slide essentially says: and what if matter had wave properties (that’s it)

Although it was my original intent to talk about (and demonstrate) the photoelectric effect quite in depth, I want to skim over it as fast as possible. I would even like to get rid of it. It is a concept that could easily take a whole presentation and I don't think it presents enough QM concepts.

As for quantum tunnelling, I admit that it will be much more that a minute. However, that time is split between the uncertainty slide and slides 24-27 that are all meant to explain tunnelling and about nothing else.

The goal of my slides is to be as simple as possible, avoiding technicalities that could lead to long tangents. I want to avoid dangerous generalisations though.

I also know that I will have three kinds of people: People who refuse to accept the theory because it seems too weird, people who accept the theory because they didn’t understand and associate QM with classical concepts, and people who are in awe and slightly confused. I would like to have more of the third kind.

And by the way guys, it may seem like I refuse to take your advice or listen to you, but talking to you guys is really helping me a lot to flesh out my presentation.

If any of you know why my experiment failed, I would love to know why. I am running out of time to fix it.

As always, I greatly appreciate all the help you have given me and continue to give me.

 

[muppet]

The most simple problem I can think of to do with quantum mechanics would use the De Broglie relation:
\lambda=\frac{h}{p}
If you're talking about the two- slit experiment (which I would definitely make the focal point of an introduction to people without advanced maths skills or prior knowledge of QM) then you could calculate the momentum of the particles needed to produce a particular interference pattern- use n\lambda=dsin\theta like in a standard GCSE problem to work out the wavelength, then de broglie to work out the momentum of the particle.
I would structure your talk as something like:
Presentation of the two slit experiment.
De broglie's explanation of 'pilot waves'
Problem
Modern understanding of what the wave is.
Although even that might push 15 mins tbh!
I know you've mentioned your teacher on here before...so be prepared to answer the questions that everyone who finds the two-slit experiment counter-intuitive to the point of offence asks: could the electrons just be bouncing off the slits, or reacting with each other in a particular way, and similar nonsenses! Be prepared (ideally) to cite specific experiments that he can go away and read about in answer to any questions you get asked.
Good luck

 

[ΔxΔp≥ћ/2]

The most simple problem I can think of to do with quantum mechanics would use the De Broglie relation

Thank you, I will use it. I was not sure whether it was as cut and dry as it seemed or still accepted as accurate. Since you have said this, I will assume that the frequency is also equal to the energy devided by Planck's constant.

De broglie's explanation of 'pilot waves'

I really want to avoid that. (long tangent, not so fundamental)

If you're talking about the two- slit experiment (which I would definitely make the focal point of an introduction to people without advanced maths skills or prior knowledge of QM) then you could calculate the momentum of the particles needed to produce a particular interference pattern- use like in a standard GCSE problem to work out the wavelength, then de broglie to work out the momentum of the particle.

I'm sorry, I do not quite see what this proves. It seems long though.

My presentation is centered around the double-slit experiment. I just want everybody to get the backround and the the concepts to understand what we think happens.

As always, thank you very much for your imput.

 

[muppet]

I don't know what country you live in or how old you are; but at school I had to solve very basic problems based on interference between two coherent sources- usually light being shone at a screen with two slits in! I may have been wrong about it being GCSE, as I can only find A-level papers on it now- see question one on http://www.aqa.org.uk/qual/gceasa/qp-ms/AQA-PA04B-W-QP-JAN07.PDF . My suggestion is if your teacher wants you to work through a problem, that you take this school-level problem, in which you would be given all bar one of the parameters n, lambda, theta and d, and use simple algebra to work out the remaining one, then substitute lambda in the equation I gave earlier for h/p to rephrase the problem in terms of QM and particles.

 

[belliott4488]

FWIW, I'll toss my 2 cents in here. First, I agree completely with blechman and nrqed: less material covered carefully and well always gets a better response than more material covered too quickly. The exception is when the presenter doesn't know what he's talking about, reads a one-minute blurb, and then has no idea where to go after that - but you've already clearly demonstrated the kind of passion that makes it likely that you'll go on and on until your teacher tells you that you're out of time - that's a good thing.

As for the experiments, I have to say that although the double-slit experiment is the "canonical" starting point for QM discussions, I've always found it difficult to get across to lay audiences. I think part of the problem is that understanding even the classical phenomenon of interference requires a certain amount of effort, since it's not an obvious part of day-to-day experience, so then going to the QM version pushes the audience past the point of comprehension, so that they miss the "oh, wow" factor. That's been my problem, but maybe you can do better.

I also agree with nrqed - I hate talking about Schr. Cat - it is misunderstood more often than understood correctly. So, my choice is blechman's suggestion of the polarizers - I've never run across that as a pedagogical tool, for some reason, but I really like it! Sort of a macroscopic example of a QM effect - sort of.

Anyway, good luck! Many of us have been in this situation - trying to pass on some of the excitement we feel about a subject - but know how frustrating it can be. I once tried to explain the interpretation of Dirac's negative energy sea to a party of drunken lay people, but I suspect all they got out of it was that I was either very drunk or very excited by strange ideas (little did they know that it was both!).

 

[David_Harkin]

I don't know what country you live in or how old you are; but at school I had to solve very basic problems based on interference between two coherent sources- usually light being shone at a screen with two slits in! I may have been wrong about it being GCSE, as I can only find A-level papers on it now- see question one on http://www.aqa.org.uk/qual/gceasa/qp-ms/AQA-PA04B-W-QP-JAN07.PDF . My suggestion is if your teacher wants you to work through a problem, that you take this school-level problem, in which you would be given all bar one of the parameters n, lambda, theta and d, and use simple algebra to work out the remaining one, then substitute lambda in the equation I gave earlier for h/p to rephrase the problem in terms of QM and particles.

HAHAHA...i done some of those questions last year for revision! lol

 

[ΔxΔp≥ћ/2]

I don't know what country you live in or how old you are; but at school I had to solve very basic problems based on interference between two coherent sources- usually light being shone at a screen with two slits in! I may have been wrong about it being GCSE, as I can only find A-level papers on it now- see question one on this paper. My suggestion is if your teacher wants you to work through a problem, that you take this school-level problem, in which you would be given all bar one of the parameters n, lambda, theta and d, and use simple algebra to work out the remaining one, then substitute lambda in the equation I gave earlier for h/p to rephrase the problem in terms of QM and particles.

I don't want to hijack my own thread, but we are taught nothing but mechanics in physics class (and I've made it to physics 3, you think we would have seen something else by now. There is one class, ph. 4 that is called EM though.), this might be a "there's no chance in hell you'll get into MIT son" program or something :-) .

Back onto subject, if my problem is not very direct or not very intuitive, no one will follow and I will lose a tremendous amount of time.

I would rather calculate the wavelength of a baseball with a certain velocity (I got this problem in a wonderful textbook that we don't use :-( ). Is that ok? Is this an appropriate way to show that the wave length is very, very short, so macroscopically, we always experience particles?

I plan on demonstrating Young’s experiment to show what interference is and then later on showing what happens when one particle at a time goes through the experiment (it still interferes, so particles can't be "bumping" each other.) In general, my presentation teaches by showing experimental observations that one could hopefully easily draw conclusions from.

I also agree with nrqed - I hate talking about Schr. Cat - it is misunderstood more often than understood correctly. So, my choice is blechman's suggestion of the polarizers - I've never run across that as a pedagogical tool, for some reason, but I really like it! Sort of a macroscopic example of a QM effect - sort of.

I want to stay as far away as possible from that darn cat.

Polarizers sound interesting, but as I said, no one has any experience with physics other than mechanics. Frankly, I still do not really understand the polarizer experiment even after the efforts of members to help me.

I would like anybody who knows why my experiment with an LED light explained in post #32 does not work and how I can fix it. As mentioned earlier, it is highly probable that I am completely ignorant about some physical property because I have no experience there.

As always thank you very much guys.

 

[muppet]

The baseball calculation would be absolutely fine. You might want to repeat it for something like a speck of dust, or something else that we think of as beng really small but macroscopic, just to emphasise how small we're talking here.
I've got to admit to being impressed by that demonstration of the HUP. The only reason I can think of is that you're talking about scales which will require a fair degree of precision and steadiness-try mounting your metal pieces in a similar way to that shown in the video

 

[ΔxΔp≥ћ/2]

I've got to admit to being impressed by that demonstration of the HUP. The only reason I can think of is that you're talking about scales which will require a fair degree of precision and steadiness-try mounting your metal pieces in a similar way to that shown in the video

If only I knew how, I don't think that I will be able to replicate that exact experimental set-up (laser, focus-thingy and that box)

When I look at the video again, it seems that my slit could possibly be smaller than his??!? As stated before, I use a wrench and the slit can get so small, that I can't see through it (a sliver of light gets through though)

 

[ΔxΔp≥ћ/2]

I just had a weird idea...

Here is part of the web page on Dr. Walter Lewin on the MIT website:

Lewin's lectures at MIT are legendary. Many have been shown for over six years on UWTV in Seattle, reaching an audience of about four million people. He personally responded to all e-mail requests he received (hundreds per year) from UWTV viewers, who varied in age from 7 to 90. For fifteen years he was on MIT Cable TV helping freshmen with their weekly homework assignments. His programs, which were aired 24 hours per day, were also frequently watched by upper-class students. Lewin is the soul of PIVoT, a video course on Newtonian Mechanics with a total of 53 hours of video clips. Additionally, his 36 lectures on Electricity and Magnetism and 23 lectures on Vibrations and Waves can also be viewed from the course's web site. Finally, his special lectures given at MIT for science teachers and for middle school students can be viewed on MIT World.

If you guys cannot help me, should I e-mail him?

 

[muppet]

Two thoughts occurred to me yesterday about your experiment:
1. Could it be that because you're not using the laser and focusing thingy that your momentum is initially uncertain anyway? So you don't notice any increase in the uncertainty as a result of narrowing your slit?
2. I don't know that the HUP explains the interference minima you can see in that video; and classical results that do that have been known since the 17th century. Unless you combined it with a presentation of the photoelectric effect, your teacher will turn around and say "So light is a wave. We knew that." So you may be better off concentrating on the wavelike behaviour of matter.

 

[blechman]

Well, while I was away, HUP, you've been getting some great advice from other people, so I feel better about not responding too quickly.

Let me say a few back comments:
1) If a slide is only "10-15 seconds long", then it is not worth having! Drop it completely! Rule of thumb: design your slides to be 1 minute each. So you should plan on no more than 15 slides on the outside.

2) As has already been mentioned by muppet, I'm not sure what your version of the slit experiment proves. All waves will behave this way (it's called "diffraction") and you don't need Heisenberg to prove it - it was known about since Newton's day! It **IS** true that there is an uncertainty relation from wave mechanics, sometimes called the "classical uncertainty principle" by textbooks, which can be used to justify this behavior, but again, it's not Heisenberg (well, of course they are related, but you don't need Heisenberg to see the effect is all I'm saying).

Where the QM comes in with light is when you lower the INTENSITY of the light - then you see that there are individual "dots" on your screen proving that light must be made of particles. Then you can do Young's double-slit experiment to prove that light must be made of waves. And then your head explodes!

Unfortunately, I do not know of a way to lower the intensity of light to that extent with equipment available in a high-school classroom. Maybe someone else has a better idea?? (This is the problem when you ask a theorist for demo ideas! )

The only thing I can think of is for you to do the double-slit experiment with light (that should be easy enough) - which by itself might not be called a "quantum experiment" since it's predicted by CLASSICAL E&M, and then SAY to your audience that when the exact same experiment is repeated with neutrons rather than light, then you see the same thing, thus "proving" that neutrons are waves! Apologize that your budget does not allow for you to acquire a neutron source, but that you will be accepting donations from any skeptics so you can prove it to them!

You can also use words to describe the quantum nature of light as I mentioned 3 paragraphs up.

This has little to do with HUP (at least directly), but there you are.

 

[ΔxΔp≥ћ/2]

''1) If a slide is only "10-15 seconds long", then it is not worth having! Drop it completely! Rule of thumb: design your slides to be 1 minute each. So you should plan on no more than 15 slides on the outside.'' Most of those slides where just picture slides, transition slides or just associated with other slides. I took your advice anyways and cut down on quite a few.

''2) As has already been mentioned by muppet, I'm not sure what your version of the slit experiment proves. All waves will behave this way (it's called "diffraction") and you don't need Heisenberg to prove it - it was known about since Newton's day! It **IS** true that there is an uncertainty relation from wave mechanics, sometimes called the "classical uncertainty principle" by textbooks, which can be used to justify this behavior, but again, it's not Heisenberg (well, of course they are related, but you don't need Heisenberg to see the effect is all I'm saying).''
I have become aware of this. However, I believe that if we explain the experiment with photons, the HUP is necessary. I will say something like ''electrons and neutrons do this too''.

''The only thing I can think of is for you to do the double-slit experiment with light (that should be easy enough) - which by itself might not be called a "quantum experiment" since it's predicted by CLASSICAL E&M, and then SAY to your audience that when the exact same experiment is repeated with neutrons rather than light, then you see the same thing, thus "proving" that neutrons are waves! Apologize that your budget does not allow for you to acquire a neutron source, but that you will be accepting donations from any skeptics so you can prove it to them!''
I have already prepared the equipement to do this demonstration. Thanks for the budget joke, I will use it.

''1. Could it be that because you're not using the laser and focusing thingy that your momentum is initially uncertain anyway? So you don't notice any increase in the uncertainty as a result of narrowing your slit?''
I have two hypothesis. One is that the wavelength of the light is too long (my LED is green). The other is that the slit is still too narrow.

I have e-mailed Walter Lewin on this subject and his response was so quick that it was scary (no more than 15-30min). He has sent my e-mail to his demo guru.

As always, thanks for the help guys. I will be on vacation until next tuesday. My dad wants to run the Pheonix marathon. It will be a good time to catch up on my P. A. M. Dirac and to do homework. I will have three oral presentations and four tests to do when I get back, not to mention exams the week after. So don't expect answers to any of your posts until then. I will probably be presenting my project on quantum mechanics thursday and the one on special relativity in two classes probably the same day.

 

[muppet]

Neither of your hypotheses works I'm afraid. Diffraction works best when the size of your slit is comparable to the wavelength of your light (and here we're talking a few hundred nanometers). So a long wavelength and a narrow slit should actually help! The only reason you see red light used in these experiments so often is because that's the kind of laser it's cheapest to make.