# B What are some easy ways to show quantum mechanics

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1. Dec 21, 2016

### malcome123

hi I'm in grade 9 and I have to make a 20 slide slideshow and a 6 page essay or a module I need ideas on what to do

2. Dec 21, 2016

### Staff: Mentor

Last edited: Dec 21, 2016
3. Dec 21, 2016

### ZapperZ

Staff Emeritus
Take your LED Christmas lights, plug it in, and you're done.

Zz.

4. Dec 21, 2016

### ChrisisC

What country do you live in? You are learning of Quantum Mechanics in the 9th grade? I live in the United States and i learned about extremely easy earth science... im interested in what you are doing learning such advanced material at such a young age.

5. Dec 21, 2016

### vanhees71

The explanation of the photo effect in the above cited Wikipedia article is utterly wrong!

6. Dec 21, 2016

### mikeyork

One of the most profound triumphs of QM is that it enables an explanation of chemistry and life as a purely physical consequence of atomic structure.

7. Dec 21, 2016

### Staff: Mentor

Okay I'll remove the link. perhaps you can suggest a better reference for the OP.

8. Dec 21, 2016

### Staff: Mentor

Here in Australia we learn very basic QM in grade 9 and 10 science - and at that time we started school a year earlier than the US ie first grade at 5 years of age. We learnt more in grade 11 and 12 - at about the level of AP physics in the US. That's why we have 3 year Bachelors in Australia and the UK - our 11 and 12 is roughly equivalent to AP level or IB SL level which is first year university in the US. In Britten they have A levels in their equivalent of 11 and 12 which is above AP or IB. In Australia ours is roughly equivalent to British AS levels. Either way we start at about US second year level in our universities which is why we have 3 year degrees. Another example is calculus - we start immediately on Multivariable calculus and advanced single variable calculus because we do calculus in HS.

Thanks
Bill

9. Dec 21, 2016

### Staff: Mentor

It will also cover quite a bit of what you will learn in 11 and 12 but that's OK.

Of those I personally would recommend Feynman who if I remember correctly was delivering it to an audiene of HS students in NZ - but dont hold me to it.
https://en.wikipedia.org/wiki/QED:_The_Strange_Theory_of_Light_and_Matter

But there are a few to choose from.

Thanks
Bill

10. Dec 21, 2016

### ChrisisC

I would have to say i am quite jealous! I would do anything to learn even the slightest quantum mechanics in high school. You Australians are doing it right let me tell you that.

11. Dec 21, 2016

### Staff: Mentor

Its specified in a general way in the Australian science curriculum. Exactly what is taught is left rather open.

We have specialist science schools like the Queensland Academy of Science where it would be taught at about the level of the books I gave. They do an IB program in 11 and 12 where it would be taught in greater depth - they even have access to university subjects.

So it varies a lot depending on the school - but the best would teach it at about the level of the books I mentioned to 14 or 15 year olds.

There are schools in the US like the Basis schools that teach at an even higher level - but they are the exception rather than the rule. It requires dedication on the part of the student that only some have.

Thanks
Bill

12. Dec 22, 2016

### vanhees71

Hm, that's a very difficult task. I've not seen yet a correct reference about the photo effect understandable at 9th grade high school. I think on this level you can only state the phenomenological facts and then just mention that in theoretical physics that's only described by quantum theory of the bound electron; no photons needed, i.e., it's on the level of undergraduate non-relativistic quantum theory; it belongs in the lecture QM 1 when you treat time-dependent perturbation theory; see my Insights article

https://www.physicsforums.com/insights/sins-physics-didactics/

13. Dec 22, 2016

### ZapperZ

Staff Emeritus
I'm the LAST one to defend a Wikipedia article, but nothing here makes what was linked to "utterly wrong". Your insight description decided to pursue this using "non-photon" picture. Fine, we know that can arrive at the naive photoelectric effect. But this also does NOT make the standard picture that we use to describe the photoelectric effect to be "utterly wrong". This was not proven to be so in your article. You offered an alternative description, not a falsifying description.

The standard photon model is used in practically all photoemission texts. See "Photoemission Spectroscopy" by Hufner (Springer), which is a well-known text for those of us who WORK in this field. The Spicer's 3-step model makes use of this picture, and it has been extremely successful in describing the microscopic process of photoemission. Heck, such a picture has been extensively used in practically ALL photon-electron emission model (see http://server2.phys.uniroma1.it/gr/...ON_SPECTROSCOPY_Mariani-Stefani_revised10.pdf).

It is not "utterly wrong". It is simply a matter of tastes.

Zz.

14. Dec 22, 2016

### Staff: Mentor

Also there's the lightandmatter.com online books by Benjamin Crowell which are used to teach at community colleges and highschools in particular this one called Simple Nature has a good writeup on Quantum Mechanics:

http://lightandmatter.com/html_books/0sn/ch13/ch13.html [Broken]

Last edited by a moderator: May 8, 2017
15. Dec 22, 2016

### vanhees71

Any source that claims a photon is like a little "billiard ball" is utterly wrong. A photon is a massless quantum of spin 1 and thus has not even a position observable! The standard model of the photon is QED and nothing else! As my Insights article shows, the "Einstein formula" is indeed not wrong, but it can be derived from the modern theory, and one should not impose wrong pictures on students. I had a hard time to unlearn these wrong pictures during my studies of physics later.

16. Dec 22, 2016

### ZapperZ

Staff Emeritus
Did the Wikipedia article explicitly stated that photons are "billiard ball"? (I wouldn't put it pass a Wikipedia article to say that) Did the sources I cited explicitly stated that?

Zz.

17. Dec 22, 2016

### Staff: Mentor

We are getting a little off topic here and far beyond the 9th grade level.

So instead can we find a credible source of information on the photo-electric effect suitable for a 9th grader?

I checked the wikipedia article briefly and could find no mention of billiard balls. So if you guys could vet the article for accuracy and post here that would be great.

Jedi

18. Dec 22, 2016

### vanhees71

I've not looked at your sources in much detail, and of course I'm pretty sure neither states it in this way, but the Wikipedia article is the usual wrong introductory treatment of the photoeffect, you find even in many textbooks on introductory quantum mechanics at the university level, and that's what I try to fight against. It's bad, because particularly starting a new subject (and even such an attractive one as QM!) leads to a strong foundation of these wrong pictures in a student's mind (at least it was the case for me), and then you study for some semesters physics, and then you learn that you have to unlearn these wrong pictures (among them the here discussed photoelectric-effect treatment and the Bohr-Sommerfeld model with its electron trajectories), but @jedishrfu is absolutely right in saying that we getting more and more off-topic.

19. Dec 22, 2016

### ZapperZ

Staff Emeritus
I hate to say this, but it was a mistake to remove the Wikipedia link. I find nothing "utterly wrong" with it (and I'm someone who works in the field of photoemission and photocathdoes). If you find the way it is written now is "utterly wrong", then many of our standard textbooks are also "utterly wrong", and you're telling people to NOT pay attention to them, contrary to the PF policy.

This isn't an issue with photoelectric effect. It is an issue of how to represent light. I've seen similar stuff being done carelessly in QFT and Feynman diagram representations involving light interaction.

Zz.

20. Dec 22, 2016

### ZapperZ

Staff Emeritus
If you care so much about giving the "wrong pictures in a student's mind", then you need to go back and re-read the IMPRESSION you left in your Post #5:

You gave no indication on where it went "utterly wrong". Thus, you are dismissing the ENTIRE article, which in fact, contains many accurate and standard description of the photoelectric effect, including the Einstein model!

Now sit back, and figure out the 'wrong picture' that you've given off to students with that kind of a post. And all because of what? That you thought the article was using "billiard balls" model, which it didn't?

Zz.

21. Dec 22, 2016

### vanhees71

Well, let's look at the quoted Wikipedia article. The first two sentences read
The first sentence is correct, of course, but the second one states that electromagnetic radiation "is made of a series of particles called photons", and already harm is done. It's utterly wrong, and I stand to this claim.

Electromagnetic radiation are, from the point of view of quantum physics, coherent states, and this is a state, where the "particle properties" (to be understood in a very loose sense either!) are not very apparent: Not even the photon number is determined, i.e., not even in a very loose sense the particle picture is appropriate.

is also wrong since the phenomenology of the effect discussed on the level of the article (and in Einstein's famous article of 1905) has turned out NOT to have anything to do with the quantum nature of light. It's true that it has to do with the quantum nature of electrons (see the standard derivation in my Insights article, which can be found in many serious textbooks on QM of course). It's fully explained as the transition from a bound state of the electron hit by classical electromagnetic waves.

The first paragraph of the section "Mechanism" is fine. The rest is bad again for the reasons discussed above. The History section sounds accurate.

22. Dec 22, 2016

### vanhees71

I've written, what's "utterly wrong" with this picture (instead of billiard balls I should have written particles), and of course Einstein's photoelectric article is wrong from the point of view of modern physics. That's nothing against Einstein, who opened the door to the modern picture with his work on quantum theory, and he participated also strongly in formulating the modern theory, pushing its development and physical understanding forward in a great way. Still, his "old quantum theory" is now substituted by the more consistent and comprehensive modern quantum mechanics (and QED when it comes to photons, where they are really needed!).

Of course, you can't teach QED in highschool, and at the same time you should tell the students some modern physics. You can do this only in a qualitative way anyway (with some very simple examples concerning the Schrödinger equation like a particle in a box and the harmonic oscillator, which we learnt in highschool, but only in the last year (in Germany 13th grade)), and it's (in my opinion) as well possible on this level to explain the photoelectric effect without using the classical-particle picture for photons.

I'd rather start with Planck's black-body radiation, because here indeed you need the quantum nature of the electromagnetic field, and it establishes a much more appropriate picture about photons. Again, you cannot give the full mathematical analysis, but you can say that Planck's work showed that if you have an electromagnetic wave of frequency $f$ it exchanges energy with matter (Planck used a harmonic oscillator as a model) in portions $E_f=h f$, which we call photons. At the same time the electromagnetic wave has a wave vector $\vec{k}$ with $k = |\vec{k}|=\omega/c=2 \pi f/c$, and the photon accordingly carries a momentum $p=\hbar k=h f/c$.

What happens in the photoelectric effect is indeed this: You shine with classical (!) light on the material, and the bound electron absorbes the energy of a photon out of the classical (!) electromagnetic wave, and the energy balance indeed reads
$$E_{\text{kin}}=h f-W,$$
where $W$ is the binding energy of the electron.

Although this is of course still not the full story, it's in my opinion better than a naive classical-particle picture of a photon. At least it's nothing qualitatively wrong.

Last edited: Dec 22, 2016
23. Dec 22, 2016

### malcome123

24. Dec 22, 2016

### Lautaro

Hi.

Why not trying an EXPERIMENT!
Photoelectric effect.

Enjoy.

Best.

Lautaro Vergara

25. Dec 22, 2016