# B What is a wave

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1. Mar 8, 2016

### Aman Trivedi

I'm going to begin studying about waves and sounds. But before I start I wanted help in understanding what really a wave is, I searched on google and YouTube and what I did understand was that waves are caused by a disturbance, due to that disturbance the potential energy gets converted to kinetic energy as energy starts moving ahead. What I don't understand is what is this wave made of? If waves and particles are different, then what exactly is a wave made of?
Thank you :)

2. Mar 8, 2016

### ZapperZ

Staff Emeritus
Do you have a textbook that is part of your beginning studies? If you do, what does it say? If you don't, how are you studying all this? Using random things you find online?

Zz.

3. Mar 8, 2016

### ProfuselyQuarky

A wave is pretty much a form in which energy is transferred without the presence of matter. There are ultimately two different types of waves: transverse waves and longitudinal wave. The primary difference between the two is that transverse waves can be polarized but longitudinal ones cannot be. A polarized wave vibrates in a single plane.

4. Mar 8, 2016

### Sturk200

In my opinion the best way to think about "what a wave is" is geometrically. In the most basic sense a wave is an abstract geometrical object that moves in some particular fashion. The wave itself is not "made of" anything. The wave is just a description of how some geometrical distortion in a medium propagates. The "wave versus particle" question isn't really interesting until you start talking particularly about electromagnetic waves. If your question is what electromagnetic waves are made of, then this is a sort of deep quantum mechanics question that nobody has a perfect answer to yet. But if we forget about that and just focus on basic mechanical waves, like sound waves or water waves or waves on a string, then the answer is straightforward: waves are propagating geometrical disturbances in a medium (moving shapes), and the medium is made of particles (presumably some kind of molecules).

Mathematically, the general representation of a wave is some function of the form f(x-vt), where x is a spatial variable and t is a time variable.

5. Mar 8, 2016

### Aman Trivedi

I've a textbook. I'm doings IGCSEs right now. The thing is that the textbook doesn't really give much reasoning behind why things are the way they are. I know how to calculate the speed, wavelength, frequencies and etc. But I don't like to learn the rules before seeing the game. :)

6. Mar 8, 2016

### davenn

well that makes it really difficult for you to learn
it would be a really good idea to start changing your mindset on that

7. Mar 8, 2016

### Aman Trivedi

Honestly, knowing the why's and how's to the what's helps me understand better. :)?

8. Mar 8, 2016

### Aman Trivedi

so energy causes distortion in a medium, and the medium carries the energy in a particular fashion similar to the initial distortion caused by energy? Another question :p what happens to sound energy after a certain distance? What does it convert into?

9. Mar 8, 2016

### Sturk200

Heat, I would think. That's to say, random motion of the surrounding air molecules.

10. Mar 8, 2016

### nasu

If you don't know the rules you won't understand the game. Especially if the game IS the rules. :)

11. Mar 8, 2016

### Staff: Mentor

I think the best explanation of a wave I've ever heard was, "A wave is that which obeys the wave equation".

12. Mar 9, 2016

### Aman Trivedi

^corrected myself lol

13. Mar 11, 2016

### ProfuselyQuarky

That's sure helpful, Drakkith

14. Mar 11, 2016

### my2cts

Any object can probably be put in this form, as seen from a moving frame of reference.
A wave is a travelling oscillation.

15. Mar 11, 2016

### sophiecentaur

Not all waves have a wavelength. because they are not always periodic. Probably an improvement would be 'a travelling disturbance'. This would then include single pulsed waves or the propagation of a 'step' transition.

16. Mar 11, 2016

### dipole

Waves are just a general term to describe the propagation of energy and momentum. Some waves are periodic and represent oscillations, some are not. They are the natural way in which energy and momentum spread out and move in continuous mediums, based on the laws of nature we happen to live with.

17. Mar 11, 2016

### Aman Trivedi

Can I think of it as a set of dominos? Where one disturbance, creates a chain travelling along?

18. Mar 11, 2016

### Staff: Mentor

Thanks. I'm here for you, after all!

19. Mar 11, 2016

### Aman Trivedi

20. Mar 12, 2016

### my2cts

The oscillation involves a displacement and a return force, which in general is nonlinear. If oscillations in one place are coupled to the next place, a travelling oscillation can result. This covers even the case of falling domino pieces. The only problem I see with this definition is that it requires a medium, so it begs the in my opinion unavoidable question: what is the medium for EM waves ?

Last edited: Mar 12, 2016
21. Mar 12, 2016

### my2cts

This transforms the question into: what is a wave equation?
Back to the drawing board :-) .

22. Mar 12, 2016

### lychette

what was a wave before the wave equation ?

23. Mar 12, 2016

### sophiecentaur

In your initial 'definition; you exclude EM by implication so it's not surprising you end up with a problem about a transmission medium.
If you acknowledge that all energy / information etc etc, takes time to travel from place to place, you have a built in reason why waves occur; it takes time for a disturbance of any kind to make its presence felt at a distance. That's all you need for a wave to form, whether it's mechanical, EM (or gravitational).

24. Mar 12, 2016

### Aman Trivedi

25. Mar 12, 2016

### collinsmark

One could argue that through Maxwell's equations, a change in the electric field causes a corresponding change the magnetic field within the surrounding region, which causes a change in the electric field which causes a change in the magnetic field and so on. It can also be shown that this resulting wave (assuming a vacuum) propagates at the speed $\frac{1}{\sqrt{\varepsilon_0 \mu_0}}$, where $\varepsilon_0$ is the "permittivity of free space" and $\mu_0$ is the "permeability of free space." No medium is necessary. This gets kinda' interesting in the perspective of special relativity, but that shouldn't come as big surprise; Maxwell's equations were one of the primary influences for Einstein writing his "On the Electrodynamics of Moving Bodies" paper in 1905. [Edit: for those unfamiliar, that was the paper that first introduced the concept of relativity.]

But I fear I'm getting off topic for the OP's question. It's probably best to start with something simpler.

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If you want to get qualitatively familiar with waves before delving into the math, try some experimentation. Wiggle one end of a rope with the other end loose. Notice the waves going through the rope. What happens when the wave gets to the other end of the rope? Does it reflect back? Now tie the other end of the rope to a wall hook or a post. Does the wave reflect back this time? Do you notice anything different about the reflected wave in each case (loose end vs. fixed end)?

Try to create "standing waves" with the rope. Is there a qualitative difference in the standing waves for a rope with the other end fixed vs. that with the other end open?

With the rope tied to the post, pull on the rope when making a wave (not a standing wave, but just a normal wave this time), thus increasing the tension on the rope. Does the tension in the rope affect the speed at which the wave travels?

Last edited: Mar 12, 2016
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