What is the difference between transverse and plane waves?

[dyn]

Hi.
Are all transverse waves plane waves ?
Are all plane waves transverse ?
I'm confused about the difference. I know the difference between transverse and longitudinal waves but I'm not sure on how plane waves fit into the picture ?
Thanks

 

[sophiecentaur]

Rather than just telling you the answer I will ask you what defines a Plane wave and what defines a Transverse Wave (and also a Longitudinal Wave)?
Why, for instance, could you not expect to find a plane sound wave? These terms all have distinct meanings and their meanings tell you all about where they are appropriate to use.

 

[dyn]

An answer would have been nice

 

[berkeman]

An answer would have been nice

I think @sophiecentaur is just trying to get you to articulate what reading and research you have done so far. I think your questions in this thread are good ones -- it would just be good if you showed us what you have found so far. Thanks.

 

[dyn]

I know the definitions of transverse and longitudinal waves and understand them. I know the definition of plane waves ( waves whose surfaces of constant phase are infinite planes perpendicular to the direction of propagation ) but I don't really understand this definition as it just sounds like a transverse wave. Hence the question.

 

[Nugatory]

Think about waves in the surface of a body of water. These are transverse waves - they're propagating horizontally along the surface of the water while the water is waving up and down vertically. Draw a line along the crest of one wave and you have a surface of constant phase; it's a straight line extending out to infinity you have a plane wave (although it's a wave in the two dimensional surface of the water instead of three-dimensional space, so the surface of constant phase is a line instead of a plane).

 

[sophiecentaur]

I know the definitions of transverse and longitudinal waves and understand them. I know the definition of plane waves ( waves whose surfaces of constant phase are infinite planes perpendicular to the direction of propagation ) but I don't really understand this definition as it just sounds like a transverse wave. Hence the question.

I asked you the question rather than giving you a simple answer for a good reason. From your post here, you already know the answer if you ask yourself why a longitudinal wave should not have constant phase over a plane (i.e. what's so special about a transverse wave that only it can have constant phase over a plane?)
PS I often get shot down when I answer questions directly - in my enthusiasm to 'educate' - and someone will intervene with a 'question'. A chap can't win.
But the point is, have you a notion of the answer to your question now?
The wave equation
A = A₀Sin(ωt-kx)
doesn't specify the direction of the displacement (polarisation or any vector direction)

 

[dyn]

Thanks for your replies but I'm still confused.
A longitudinal wave can not be a plane wave ?
A transverse wave on a string can not be a plane wave as there will not be a surface of constant phase only single points ?
Any transverse wave in 2-D or 3-D will be a plane wave as long as the surfaces of constant phase are infinite planes perpendicular to the direction of propagation eg an EM wave ?

 

[sophiecentaur]

Thanks for your replies but I'm still confused.
A longitudinal wave can not be a plane wave ?
A transverse wave on a string can not be a plane wave as there will not be a surface of constant phase only single points ?
Any transverse wave in 2-D or 3-D will be a plane wave as long as the surfaces of constant phase are infinite planes perpendicular to the direction of propagation eg an EM wave ?

If the phase of a wave is the same over a plane, it is a plane wave, what has the phase got to do with the direction of displacement of the oscillations? Have you been reading what has been written above in this thread? Perhaps you could tell us where you are finding contradictions. Are you using the right definitions for the terms? Did you look them up?
The practicality is that a true plane wave is hard to produce and it is best to start with a small ('point') source and let a spherical wave expand far enough to consider a part of that surface as being a plane. (The same as we treat the ground under our feet as flat, despite having a spherical Earth.)

 

[dyn]

If the phase of a wave is the same over a plane, it is a plane wave, what has the phase got to do with the direction of displacement of the oscillations? Have you been reading what has been written above in this thread? Perhaps you could tell us where you are finding contradictions. Are you using the right definitions for the terms? Did you look them up?
The practicality is that a true plane wave is hard to produce and it is best to start with a small ('point') source and let a spherical wave expand far enough to consider a part of that surface as being a plane. (The same as we treat the ground under our feet as flat, despite having a spherical Earth.)

Yes I have been reading what has been written. That's the point of a forum. Have you ever considered that the fact that as I obviously don't understand this issue ,the problem might lie with the clarity of the advice given to me ?

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Based on your previous post in a different thread, this thread will remain closed.

hi strangerep. If you don't want to answer my question that's up to you. I have a policy of only accepting help from people who wish to help me.