The Nature of Light

[Sigurd]

I need an example of progressive knowledge and I believe such a reference was made here at the .org about light.

By this I mean - in science classes in schools, pupils are taught about the nature of light. First the basics like particle and wave, later they have to unlearn some of it and fill in new knowledge.

Are there anyone here who remembers such a step by step knowledge ladder with light in particular?
I know I have read about this somewhere and it involved at least three steps if not more.

[cmacq]

Sigurd,

please read this thread

https://forums.totalwar.org/vb/showthread.php?t=103584

You wouldn't find any answers other than those relating to your questions there? It starts near the end of the first page.


CmacQ

[Sigurd]

Hi CmacQ,

It was the first thread I checked. Alas, the info I seek is not located there.
The Big bang thread discusses light on university level if I am not mistaken.

What I am looking for is to make a milk before meat representation of how we as students progress in knowledge during our school years.
I distinctly remember someone making this representation using the nature of light as an example.

By this I mean:
In secondary school we learn that light is X, but in college we learn that light is X and Y. But in university we learn that light never was X but rather X, Z and G. (this is not how it is, but you get the idea).

[cmacq]

Are you referring to Wave–Particle Duality, Special Relativity, Quantum, Quantum Electrodynamics, or String Theory?

[Sigurd]

Are you referring to Wave–Particle Duality, Special Relativity, Quantum, Quantum Electrodynamics, or String Theory?
I guess all the above.

Do you remember what you were taught first? Was it only about light being particles?
Then later in a more advanced class you learned that light also had wave attributes. From there it snowballed...
Mind you I never took any science classes at university. The last physics class I had was in secondary school.

[Reenk Roink]

Well I can't be of much help with your light example (I didn't take physics in high school, only college), though if I remember correctly, light was classically thought to be 'wave' in nature (don't know if this means that it was first taught as a wave and then the particle properties were brought up later - I was taught that it exhibited both properties from the start).

There are some other examples you can use that follow the progression you are looking for however. Classical Mechanics which I'm almost certain is taught (in high school) before Quantum Mechanics would be a good one (even in a Physics program in college, you start with classical mechanics and then progress).

Perhaps another example though somewhat more complicated is Valence Bond Theory and Molecular Orbital Theory when discussing why covalent bonds form.

[Sigurd]

Well I can't be of much help with your light example (I didn't take physics in high school, only college), though if I remember correctly, light was classically thought to be 'wave' in nature (don't know if this means that it was first taught as a wave and then the particle properties were brought up later - I was taught that it exhibited both properties from the start).

There are some other examples you can use that follow the progression you are looking for however. Classical Mechanics which I'm almost certain is taught (in high school) before Quantum Mechanics would be a good one (even in a Physics program in college, you start with classical mechanics and then progress).

Perhaps another example though somewhat more complicated is Valence Bond Theory and Molecular Orbital Theory when discussing why covalent bonds form.

Yes, it doesn't really have to be light. But I need a common basic concept to start off with.
The audience would probably be lost with too scientific stuff. All though, for effect - the last step or steps should be new knowledge to the audience - showing that they might be somewhere along the progression line but not at the end.

Someone did a beautiful progressive knowledge model using light as the example.
I thought it was a member of this site.

To make my example genuine, I need to make the correct knowledge steps but I can't really find what pupils are specifically taught in the different levels of education.

It has been too long since I sat on a school bench. I had hopes that some of you good orgahs who currently work in or take education could help me out here.

[ajaxfetish]

I think I learned about it being a wave first, and I suppose that's the oldest interpretation. Weren't studies with prisms and such a feature of Newtonian-era science? It was in Jr. High that I was first taught about the particle nature of light in any depth, and about wave-particle duality, though I think I'd become familiar with the term 'photon' earlier. If there's something beyond particle/wave, and I suppose there probably is, I never went that far in the hard sciences.

Ajax

[Sigurd]

I PM'ed Poor Bloody Infantry also, as he was one of those I thought had posted something like this.
He responded with a bullseye and asked if it would be OK to post it here also.
I answered that that would be a good idea. Apparently he has been busy since, and I will take the liberty to post his comment here:



Basically, up to GCSE level (school exams taken at age 16) one is taught about light as being a "ray" which always travels in straight lines and obeys Fermat's principle (http://en.wikipedia.org/wiki/Fermat%27s_principle), that light always takes the shortest path in terms of travel time. This is basically pretty close to the Newtonian view of light as understood in around the 18th century, and is sufficient to explain most of classical optics such as reflection and refraction up to calculating such things as the focal length of a lens.

At A-level (exams taken at age 18) one is introduced to the concept of diffraction, whereby the path light is deflected as it passes close to an object, most commonly illustrated by passing a beam of light through a narrow slit. This is impossible to explain in terms of light being a ray or a classical particle but is on the other hand a characteristic property of waves. Historically this led to the development of the wave theory of light and specifically Huygen's principle (http://en.wikipedia.org/wiki/Huygens%27s_principle) whereby one considers the propagation of a light wave by considering a series of "wavelets" emanating from every point along a wave front and examining in which directions the wavelets will cancel one another, and in which directions they will reinforce. This is the approach taught at A-level, and allows one to correctly describe diffraction in addition to reproducing all the results of the GCSE approach, i.e. refraction and reflection.

At University level we covered most of this material again in my 1st year from a more formal, mathematical perspective, but we were also introduced to the paradoxes in the classical (wave theory) view of light which led to the development of quantum mechanics. The main issue is the conflicting classical view of light being either a stream of particles or a continuous wave, with sound experimental evidence for both (such as diffraction indicating light must be a wave, while the photoelectric effect (http://en.wikipedia.org/wiki/Photoelectric_effect) suggests light must travel in discrete packets of energy). Quantum mechanics resolves the issue by describing light as photons, which in some ways behave like waves, in some ways like classical particles, but also with some entirely new properties not predicted by classical physics. This leads on to the contemporary field of Quantum Optics, which I studied briefly in my final year and covers things like entanglement, quantum cryptography and quantum teleportation.

At postgraduate level the generally accepted model is Quantum Electrodynamics, which is basically quantum mechanics fused with special relativity. As far as I am aware this model accurately describes almost all properties of light so far discovered (though it must be combined into a larger theory if one wants to describe other properties of matter such as nuclear physics or gravity).

I suppose in a way the order in which different theories of light are taught loosely mirrors the historical development of the theories; one starts with a relatively simple model of light which explains the known properties of light, and the model is replaced with successively more sophisticated models to explain new properties of light as they are discovered. It used to annoy me, that almost every year we were told "Okay, everything you've learnt up to now about light is wrong, here's how it actually works", but I realise now it is designed to help you get used to the idea that you are not trying to comprehensively describe how light "works", since no one really knows; you are simply drawing some analogy to see what properties of light it predicts. I have found this is an approach we use very often in research, frequently switching between different particle physics models which explain a certain aspect of physics (and very often only that aspect) for calculational simplicity. I guess the point is that if all you want to do is calculate lens focal lengths, you don't need to learn quantum mechanics.

[Beskar]

Yes, that looks right. It has been around almost 6 years ago since I did this in 6th form. Was going to mention about photons and narrow slits experiment.

[cmacq]

Right, the only element missing is how String Theory impacts on Quantum Electrodynamics. Thats when things get interesting.

[PBI]

I deliberately left out string theory since:

1) It is currently untested (and in fact untestable) and therefore more along the lines of a group of interesting potential lines of enquiry rather than a single coherent theory, and

2) It is primarily an attempt to resolve issues in quantum gravity rather than some unexplained aspect of light. As far as I am aware QED already provides a comprehensive description of the latter to within current experimental precision.

I did try to just give an outline of the increasingly sophisticated models of light rather than a full overview of the development of modern physics, although I seem to have ended up covering rather a lot of it anyway.

[Aemilius Paulus]

Yep, nor is the string 'theory' a 'theory'. Just an untested hypothesis.