# Light as a wave

1. Mar 9, 2013

### Johnahh

I am currently studying AS level physics and we have just started the waves module.
When describing light as a WAVE our teacher is telling us that particles are oscillating 90 degrees to the propagation of the wave.
What is acting at right angles to the propagation of the wave? As far as i was aware the amplitude of the wave was the strength of the electric and magnetic field and 45 degrees to each other?

2. Mar 9, 2013

### DennisN

The electric field oscillates perpendicular to the direction of propagation. The magnetic field oscillates perpendicular to the direction of propagation, and perpendicular to the electric field (perpendicular = 90°). See this link for a visualization (I also should point out this is an idealized model of light).

Last edited: Mar 9, 2013
3. Mar 9, 2013

### sophiecentaur

There are no "particles" oscillating in EM waves, The transverse 'displacement' is in the form of varying E and M Fields. Mechanical waves have particles, however.
That's 90 degrees, not 45 degrees and they are, also, each at right angles to the propagation direction of the wave. Look at the pictures in your book or here.

4. Mar 9, 2013

### Drakkith

Staff Emeritus
An EM wave impinging on an antenna will cause the particles to oscillate 90 degrees perpendicular to the direction of propagation, correct sophie? Perhaps that is what the teacher meant. Hopefully at least, as I hope they didn't mean that photons are oscillating up and down in the EM wave itself...

5. Mar 9, 2013

### sophiecentaur

Mmm. I think you're being a bit generous there, Drakkith. In any case, why involve particles in antenna theory? People just don't do that.
It was more likely a misunderstanding. The 45 degree bit was strange, though.

6. Mar 9, 2013

### Drakkith

Staff Emeritus
Dunno.
I guess I wasn't sure how else to describe how an EM wave interacts with matter in this context.

7. Mar 9, 2013

### Johnahh

Thanks for the replies.
He was not talking about antennas just light, I knew he was teaching it wrong.
My mistake I did not mean 45 degrees I meant perpendicular.

8. Mar 9, 2013

### Drakkith

Staff Emeritus
Indeed, there are no photons or other particles oscillating in an EM wave. It is the electric and magnetic field vectors which oscillate back and forth.

9. Mar 9, 2013

### Johnahh

Thanks Drakkith.
When talking about light as photons how does one show this graphically? is a displacement/time graph still used? and what would it look like?

10. Mar 9, 2013

### Drakkith

Staff Emeritus
One does not show it graphically, as there is no need to. Light is an EM wave. It travels as such. Photons only show up when EM waves interact with matter as little packets of energy. Light is not made of little photons zooming around like bullets. Quite simply, a photon is the quantized interaction of an electromagnetic wave with matter.

11. Mar 9, 2013

### sophiecentaur

For a constant level of illumination, the rate of arrival of photons would be constant and a horizontal line graph wouldn't be very informative. Photons do not 'wiggle from side to side' or any such nonsensical animation.

The wave and photon alternatives are a bit like the monsters the hero encounters in the adventure movie. He first meets the relatively harmless monster (waves) which he eventually manages to defeat. Then he turns the corner and meets a much bigger and generally nastier monster (photons) which will surely destroy him unless he's already dealt with the slightly less dangerous waves monster. he will never get the gold if he tries to mess with the photon monster first, believe me.
You really must try to avoid any more than a very superficial use of photons in your introduction to EM energy until you are really familiar with the wave stuff. At School, they tell you light is 'made up of' photons but so many teachers haven't got a clue what that means. You are really lucky if you find a teacher (even for A level) who can give you the real gen on photons. The way photons are presented to you tends to be as if they are little bullets. This is so wrong you wouldn't believe it!!!
The wave model, otoh, works like a treat for nearly everything you will come across - except for the photoelectric effect, radioactive decay and the 'hydrogen atom'. I am pretty certain that you will not find any exam questions that ask for a knowledge of photons at any greater depth than the very basic idea of quantisation and to know the energy and momentum associated with a photon. You will, however, need to know about the basics of waves, definitions, interference, diffraction, phase etc. etc. for light, sound and radio waves.
Get that stuff sorted out as well as you can and do a minimum of worrying about photons - seriously.
I can almost guarantee that, with the model of photons that you will get at your level, you will find it impossible to relate that model to anything but the simplest situations - anything else will lead you into loopy conclusions and apparent paradoxes. The National Curriculum doesn't seem to recognise this, I'm afraid.

12. Mar 9, 2013

### Johnahh

Sophie, I must admit the education system in England is AWFUL, I wish to learn of superposition and other QM theories yet i am taught incomplete classical mechanics which discourages me to work. i have read QED by Feynman, amazing book with so much insight the guy is a genius and E=mc^2 by brian cox. all i wish to do is learn QM lol.

13. Mar 10, 2013

### sophiecentaur

Possibly 'inadequate' but not necessarily "awful". If there's anyone to blame, I would suggest the Great British Public and recent history for the responsibility to have been transferred from shared Learning (student activity) and Teaching (Teacher activity) to Entertaining Presentation (Teacher requirement only). Having seen some of the presentations on You tube from other countries, I could say it is far from the worst!!
The problem is that the National Curriculum (Science) seems to have been arrived at by a combination of Politicians who don't understand Science at all, Science Experts who think it is just an easy, fascinating subject and Media people who just want to hype everything up. Few of the parties involved seem to acknowledge the risks of introducing so much stuff at too superficial level - which is what the present system does.
If you can really cope with QED and QM with your level of knowledge then you must have been reading and 'understanding' well outside the Specification. But, in view of your question about waves, I doubt that. Without a good Post A Level knowledge of Maths and the basics of Waves, Mechanics and EM the 'Modern Physics' topics are going to give you problems sooner or later (Science is a very heirarchical field). It's essential not to dismiss all that stuff just because what you read of QM and what Feynman has to say is entertaining. That Feynman guy certainly knew all the basics before he ever got into the stuff he's renowned for and his grasp of those sexy things is so good that you can easily be fooled into thinking that you've got it too.
I got very cross with the recent AS Physics course which was introducing Quarks before even explaining to students the meaning of the Electron Volt and the Photoelectric Effect. Glib or what??
But, if you have the motivation to progress this stuff on your own - great; keep it up. Even better if some of your peers have the same idea.

14. Mar 10, 2013

### Johnahh

I know theories and concepts of some areas of QM but none of the maths, I have no idea about tensors or matrices. I have been trying to find a book to read on QM which would make some sense but have been unable to find one.
Could you recommend a book on QM and on light that will broaden my knowledge? light is very interesting.

15. Mar 10, 2013

### Drakkith

Staff Emeritus
The only book that will really help you understand the math and core concepts of QM is an actual textbook. Everything else just sugarcoats it with analogies in an attempt to help people who have no concept of the math behind QM understand it. The only reason I have any inkling of how QM works is through several years of browsing through the forums and reading all kinds of threads on QM. And of course the help of a few people like Sophie who tore through my incorrect concepts like a sledgehammer through wet tissue paper. The cheap stuff too.

16. Mar 10, 2013

### Johnahh

yeah I have only read popular science books about QM and things on the internet.

You made a funny! so I guess i need to find a textbook to read over summer. Im guessing i will need a book on the maths used to accompany it too.

17. Mar 10, 2013

### Emilyjoint

If you are studying in England you surely must have a text book to make up for the inadequacies of your teachers. The AQA text books by Breithaupt are perfect for the level you are studying. EM waves and photons are explained perfectly with clear diagrams of EM waves.
Past exam papers and solutions are also readily available from the AQA website so you can see good explanations.

18. Mar 10, 2013

### DennisN

In addition to the good advice from Sophie, Drakkith and Emily, I'd say the following areas are particularly important as a physics foundation if you are interested in light and later quantum mechanics;

* Classical Mechanics, Wave Mechanics (classical)
* Optics
* Electricity & Magnetism -> Maxwell Equations

(links are examples of what I mean)
Sadly I have no particular books to recommend; it was a long time ago I studied myself, and when I did I used Swedish literature . But I also recommend textbooks, there's where the meat is.

19. Mar 10, 2013

### sophiecentaur

Jim Breithauf is particularly good - especially in his Understanding Physics Text book. I have had 'good' A Level students a bit at a loss with the his Course Guides / Abbreviated Books (@ about £10) which have passages which are not as explicit as in the big book - but that's about £40 iirc. I just hate books that tell you just what you absolutely 'need to know' for the purpose of the exam.
I have much more of an issue with the course structure, though. The board have no excuse as they know, full well, how little the GCSE Double award contains but they mortgage the future by trying to make the early topics sexy and up to date, without getting a bit of basic rigour in at the start. Anyone would think that Physics has got easier instead of harder by the year. Come back Nuffield Physics!!!

20. Mar 11, 2013

### Johnahh

I am doing the OCR physics and the text book is useless. it just tells you the basics for the exam and no real explanation, a lot of the information in it i find useless its only used for definitions and formula. also my teacher is a good teacher, very nice explanations but he only really knows what he needs to, I cant really expect an answer about a problem not in the curriculum.

Thank you for these links dennisN i will have a look through hyperphysics.

Sophie i looked up Jim Breithaupt he has a few understanding physics book, which one were you referring too?