# waves

Watch
Announcements
#1
this is acc my least favourite topic cuz I just don't understand wtf a wave acc is lol. id really appreciate it if someone has any tips or resources that can help me understand eh fundamentals of a wave. thanks sooo much

some of the things I'm confused about:
- sound waves,lonigtudinal, are mechanical waves right? so they need particles to move. But don't all waves need particles to move? isn't that why we have longitudinal and transverse waves?
- in transverse waves, the particle oscillate perpendicular to the direction of energy transfer right?? so they move up n down. so how does the energy actually transfer from left to right

see its stupid things like that that i don't get...
0
10 months ago
#2
Those are really good questions, which show that you are trying to engage seriously with the underlying concepts rather than simply apply formulae mindlessly. Having said that, at GCSE or even A Level, you can probably get away with a bit of rote learning at first while developing a deeper understanding of the concepts, so don't let this hold you back too much.

Your two questions are actually related. You are right that all waves need "particles" (more generally described as a "medium") in which to move (or "propagate"). This is true even for light, although the medium there is rather a special one which has no absolute reference frame (which is another story...).

For sound waves, the medium is usually air (but in general it could be any solid, liquid or gas), which is a collection of molecules moving around at high speeds with very low density. For a wave to propagate (move), energy has to be transferred from one group of molecules to the next. This is very difficult to do by transverse motion, because the viscosity of air is very low. Try to extinguish a candle by moving your hand up and down next to it (without pushing air towards it) - you will have a hard time.

On the other hand, air exerts a non-negligible pressure by virtue of the fact that the molecules are moving at high speeds. Air can be pushed from one place to another, and it has a certain 'elasticity', like a solid, but only when it is confined or pushed at high speeds. This is why it can support a longitudinal wave. Try to extinguish a candle by pushing air towards it, either by waving your hand or blowing - it's quite easy to do.

In contrast to gases and liquids (which are both fluids), sound waves in solids CAN be transverse. This is because atoms in solids are usually bonded together by strong forces, and their viscosity is extremely high (effectively infinite) compared with fluids. Earthquakes, for example, have longitudinal and transverse components which arrive at different times, and can transmit huge amounts of energy. The energy is transferred by the fact that one part of the solid is rigidly connected to another in a way that couples both lateral and longitudinal motion of the atoms.

You may wonder why light is a transverse wave and not a longitudinal one. That is an excellent question, but unfortunately a full answer goes well beyond school physics into something called Maxwell's equations. Suffice to say that electromagnetic waves do not behave like matter waves, and there is no easy mechanical analogy to explain how they propagate.

That was probably far more than you wanted to know, but I thought the questions were interesting.
1
#3
(Original post by lordaxil)
Those are really good questions, which show that you are trying to engage seriously with the underlying concepts rather than simply apply formulae mindlessly. Having said that, at GCSE or even A Level, you can probably get away with a bit of rote learning at first while developing a deeper understanding of the concepts, so don't let this hold you back too much.

Your two questions are actually related. You are right that all waves need "particles" (more generally described as a "medium") in which to move (or "propagate"). This is true even for light, although the medium there is rather a special one which has no absolute reference frame (which is another story...).

For sound waves, the medium is usually air (but in general it could be any solid, liquid or gas), which is a collection of molecules moving around at high speeds with very low density. For a wave to propagate (move), energy has to be transferred from one group of molecules to the next. This is very difficult to do by transverse motion, because the viscosity of air is very low. Try to extinguish a candle by moving your hand up and down next to it (without pushing air towards it) - you will have a hard time.

On the other hand, air exerts a non-negligible pressure by virtue of the fact that the molecules are moving at high speeds. Air can be pushed from one place to another, and it has a certain 'elasticity', like a solid, but only when it is confined or pushed at high speeds. This is why it can support a longitudinal wave. Try to extinguish a candle by pushing air towards it, either by waving your hand or blowing - it's quite easy to do.

In contrast to gases and liquids (which are both fluids), sound waves in solids CAN be transverse. This is because atoms in solids are usually bonded together by strong forces, and their viscosity is extremely high (effectively infinite) compared with fluids. Earthquakes, for example, have longitudinal and transverse components which arrive at different times, and can transmit huge amounts of energy. The energy is transferred by the fact that one part of the solid is rigidly connected to another in a way that couples both lateral and longitudinal motion of the atoms.

You may wonder why light is a transverse wave and not a longitudinal one. That is an excellent question, but unfortunately a full answer goes well beyond school physics into something called Maxwell's equations. Suffice to say that electromagnetic waves do not behave like matter waves, and there is no easy mechanical analogy to explain how they propagate.

That was probably far more than you wanted to know, but I thought the questions were interesting.
honestly, thank you soooo much!!! i acc understand it more now

have a good day
0
9 months ago
#4
(Original post by lordaxil)
.....
Your two questions are actually related. You are right that all waves need "particles" (more generally described as a "medium" in which to move (or "propagate". This is true even for light, although the medium there is rather a special one which has no absolute reference frame (which is another story...).
....
Are you sure all waves need a medium to propagate?
https://science.nasa.gov/ems/02_anatomy

From the website:
Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.
0
9 months ago
#5
(Original post by Eimmanuel)
Are you sure all waves need a medium to propagate?
https://science.nasa.gov/ems/02_anatomy

From the website:
Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.
That is an interesting point - and there is certainly a difference of opinion about this, but I would disgree that "vacuum of space" is not a medium. Our most modern understanding of the vacuum state from quantum field theory is that it is not truly "empty" space: there exist virtual fields which can be stimulated by passage of electromagnetic waves. Even from a classical point of view, the speed of light in vacuum is derived from the permeability and permittivity of free space, which are definitely non-zero, and represent in some sense the "polarizibility" of free space. If the vacuum were truly "empty space" then these should be zero and the speed of light in vacuum would be infinite.

Even without considering the technical details, saying that EM waves can propagate in the absence of a medium defies the intuitive definition of a wave as an excitation in a background field. Without a field, there can be no excitation, so I believe it is bad pedagogy to say that EM waves propagate in the absence of a medium. The standard objection to this is that there is no state of rest in free space (lack of evidence for static aether): this is fine, as long as you consider the Lorentz invariant transformation rules obeyed by EM field. There is nothing in the definition of "medium" which requires it to be a static state of absolute rest.

Further reading here (takes a more nuanced position than mine: says aether may exist, but we cannot observe it)
https://medium.com/starts-with-a-ban...t-ee7257427eb5
Last edited by lordaxil; 9 months ago
0
#6
(Original post by lordaxil)
Those are really good questions, which show that you are trying to engage seriously with the underlying concepts rather than simply apply formulae mindlessly. Having said that, at GCSE or even A Level, you can probably get away with a bit of rote learning at first while developing a deeper understanding of the concepts, so don't let this hold you back too much.

Your two questions are actually related. You are right that all waves need "particles" (more generally described as a "medium") in which to move (or "propagate"). This is true even for light, although the medium there is rather a special one which has no absolute reference frame (which is another story...).

For sound waves, the medium is usually air (but in general it could be any solid, liquid or gas), which is a collection of molecules moving around at high speeds with very low density. For a wave to propagate (move), energy has to be transferred from one group of molecules to the next. This is very difficult to do by transverse motion, because the viscosity of air is very low. Try to extinguish a candle by moving your hand up and down next to it (without pushing air towards it) - you will have a hard time.

On the other hand, air exerts a non-negligible pressure by virtue of the fact that the molecules are moving at high speeds. Air can be pushed from one place to another, and it has a certain 'elasticity', like a solid, but only when it is confined or pushed at high speeds. This is why it can support a longitudinal wave. Try to extinguish a candle by pushing air towards it, either by waving your hand or blowing - it's quite easy to do.

In contrast to gases and liquids (which are both fluids), sound waves in solids CAN be transverse. This is because atoms in solids are usually bonded together by strong forces, and their viscosity is extremely high (effectively infinite) compared with fluids. Earthquakes, for example, have longitudinal and transverse components which arrive at different times, and can transmit huge amounts of energy. The energy is transferred by the fact that one part of the solid is rigidly connected to another in a way that couples both lateral and longitudinal motion of the atoms.

You may wonder why light is a transverse wave and not a longitudinal one. That is an excellent question, but unfortunately a full answer goes well beyond school physics into something called Maxwell's equations. Suffice to say that electromagnetic waves do not behave like matter waves, and there is no easy mechanical analogy to explain how they propagate.

That was probably far more than you wanted to know, but I thought the questions were interesting.
I just wanna say lol thanks to Ur help I got 94% on my physics test 😊😊 thanks for answering all my questions aha
0
9 months ago
#7
(Original post by vix.xvi)
I just wanna say lol thanks to Ur help I got 94% on my physics test 😊😊 thanks for answering all my questions aha
Great! Thanks for the feedback.
0
9 months ago
#8
(Original post by lordaxil)
That is an interesting point - and there is certainly a difference of opinion about this, but I would disgree that "vacuum of space" is not a medium. Our most modern understanding of the vacuum state from quantum field theory is that it is not truly "empty" space: there exist virtual fields which can be stimulated by passage of electromagnetic waves. Even from a classical point of view, the speed of light in vacuum is derived from the permeability and permittivity of free space, which are definitely non-zero, and represent in some sense the "polarizibility" of free space. If the vacuum were truly "empty space" then these should be zero and the speed of light in vacuum would be infinite.

Even without considering the technical details, saying that EM waves can propagate in the absence of a medium defies the intuitive definition of a wave as an excitation in a background field. Without a field, there can be no excitation, so I believe it is bad pedagogy to say that EM waves propagate in the absence of a medium. The standard objection to this is that there is no state of rest in free space (lack of evidence for static aether): this is fine, as long as you consider the Lorentz invariant transformation rules obeyed by EM field. There is nothing in the definition of "medium" which requires it to be a static state of absolute rest.

Further reading here (takes a more nuanced position than mine: says aether may exist, but we cannot observe it)
https://medium.com/starts-with-a-ban...t-ee7257427eb5

It seems there is more disagreements than agreements after reading your post.

First of all, I must say I did not study quantum field theory (QFT), so I may misunderstand what you have written. Although I have read some articles (all are pop science IMO) in regards to QFT, my “understanding” of QFT is still equivalent to null. Pop science articles tend to “misuse” some scientific terms to explain the scientific theory to the layman.

… Our most modern understanding of the vacuum state from quantum field theory is that it is not truly "empty" space: there exist virtual fields which can be stimulated by passage of electromagnetic waves. …
It seems that you are talking about physical vacuum state, if so I agree that there are real physical entity known as fields in the physical vacuum. However, to “claim” that it is not truly "empty" space with the existence of virtual field stimulated by passage of electromagnetic waves, tell me nothing as far as I “know” (I can be wrong). As in real physical process, virtual field is not measurable and is some mathematical construct like virtual particle, not something that we actually observe. IMO the more convincing evidence is the experimental demonstration of the Casimir effect. As far as I know virtual is the antonym of real and this is no difference in QFT. Perhaps, I have misunderstand the true meaning virtual field in QFT.

By “claiming” the non-zero values of the permeability and permittivity of free space “indicate” the “polarizability” of free space is bold IMO. I doubt this is the conventional way of interpreting them in classical electrodynamics or electromagnetism. Indeed, in quantum electrodynamics NOT classical electrodynamics, the physical vacuum can be polarized.

… Even without considering the technical details, saying that EM waves can propagate in the absence of a medium defies the intuitive definition of a wave as an excitation in a background field. Without a field, there can be no excitation, so I believe it is bad pedagogy to say that EM waves propagate in the absence of a medium. …
I believe “the” “intuitive definition” (polite cough..) of wave as an excitation in a background field can be found in QFT, however, this “intuitive definition” is not found most/any introductory physics text or course. As far as I know, many the introductory physics texts don’t really give a definition of a wave which I think is good. Maybe in the very near future, the author(s) of introductory physics text would better define the word medium or differentiate material medium from field medium to make a more unified “definition” of a wave.

Whenever I hear or see the terms “bad pedagogy”(polite cough..) from an unknown source or online articles without any citations, I tend to …..
I recall someone (also an author of physics textbook(s), quite advanced in age IMO) in a conference made a reply to a question from an audience (cannot really remember the question) where the “someone” said that if you want to point out something in book is of bad pedagogy, why not write an equivalent book to show that you have a better pedagogy of teaching the concept(s) or material(s) coherently. … (Not really the exact wording)

Perhaps a more easy way (IMO) is to publish a paper with a “sensational” title called "Concept that many physics textbooks or pop science articles get it wrong" to attract the educators' attention.

As this thread belongs to other, so it will intrusive for me to reply issue that is not pertaining to OP question. I would just leave as it is unless it is the OP who want to clarify something.
0
8 months ago
#9
You seem to have developed quite a nasty cough there - hope it's nothing serious!

I think it goes without saying that the discussion has now gone well beyond the OP's original questions, but since they seem generally happy with the answers, I hope they won't mind us prolonging the discussion a little more.

It is also clear that if your exam mark scheme or syllabus says that EM waves propagate in the absence of a medium, then that is what you should write if you want to get the marks. However, what concerns me below, is what is actually the case in reality.

(Original post by Eimmanuel)
It seems there is more disagreements than agreements after reading your post.
Quite so - I agree that this is not an area where there is an easy consensus.

(Original post by Eimmanuel)
It seems that you are talking about physical vacuum state, if so I agree that there are real physical entity known as fields in the physical vacuum.
Yes, that is what I meant.

(Original post by Eimmanuel)
However, to “claim” that it is not truly "empty" space with the existence of virtual field stimulated by passage of electromagnetic waves, tell me nothing as far as I “know” (I can be wrong). As in real physical process, virtual field is not measurable and is some mathematical construct like virtual particle, not something that we actually observe.
I am not really sure what you mean by that first sentence, but as you say below, the observation of phenomena such as the Casimir effect also vacuum polarization demonstrate that the presence of virtual fields can have measurable effects on real objects. The injection of energy (such as from a passing wave) into a virtual field allows it to interact with real entities.

(Original post by Eimmanuel)
By “claiming” the non-zero values of the permeability and permittivity of free space “indicate” the “polarizability” of free space is bold IMO. I doubt this is the conventional way of interpreting them in classical electrodynamics or electromagnetism. Indeed, in quantum electrodynamics NOT classical electrodynamics, the physical vacuum can be polarized.
In classical electrodynamics, the permittivity and permeability of free space are just constants, and have no fundamental meaning. However, in QED, it becomes clear that physical vacuum can be polarized, so it is natural to interpret them as the electrical and magnetic polarizability of the vacuum state. If the physical vacuum were truly empty, then they should be identically zero, and EM waves would not propagate.

(Original post by Eimmanuel)
I believe “the” “intuitive definition” (polite cough..) of wave as an excitation in a background field can be found in QFT, however, this “intuitive definition” is not found most/any introductory physics text or course. As far as I know, many the introductory physics texts don’t really give a definition of a wave which I think is good.
Don't you think it odd that an introductory text book should introduce a concept without properly defining it?

(Original post by Eimmanuel)
Maybe in the very near future, the author(s) of introductory physics text would better define the word medium or differentiate material medium from field medium to make a more unified “definition” of a wave.
I don't think there is a need to differentiate material medium from field mediums, nor matter waves from electromagnetic waves. In both cases, they result from excitation of a background field.

(Original post by Eimmanuel)
Whenever I hear or see the terms “bad pedagogy”(polite cough..) from an unknown source or online articles without any citations, I tend to ….. I recall someone (also an author of physics textbook(s), quite advanced in age IMO) in a conference made a reply to a question from an audience (cannot really remember the question) where the “someone” said that if you want to point out something in book is of bad pedagogy, why not write an equivalent book to show that you have a better pedagogy of teaching the concept(s) or material(s) coherently. … (Not really the exact wording)
Well, I don't want to be "that someone" in the audience, but it seems to me that there is no need to produce new textbooks, simply stop pretending that EM waves are different to matter waves and "propagate in absence of medium" which is in contradiction to their wavelike nature.

(Original post by Eimmanuel)
Perhaps a more easy way (IMO) is to publish a paper with a “sensational” title called "Concept that many physics textbooks or pop science articles get it wrong" to attract the educators' attention.
Several others have done this, for example here and here.

0
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### How are you feeling about starting university this autumn?

Really excited (58)
22.39%
Excited but a bit nervous (117)
45.17%
Not bothered either way (34)
13.13%
I'm really nervous (50)
19.31%