# Tag Info

## Hot answers tagged waves

5

### In terms of the Doppler effect, what happens when the source is moving faster than the wave?

The first image shows an object traveling at Mach 1 ($v=c$). The second one shows the object traveling at some supersonic velocity ($v>c$). For both the cases, the longitudinal pressure waves pile up. Say the observer is standing in the ground and the object is traveling at $c$. The observer can't hear the pitch of sound because, the waves reach him ...

5

### De Broglie wavelength, frequency and velocity - interpretation

Yes the product $\nu \lambda$ makes sense as a velocity. Defining $E = \hslash \omega$ and $p=\hslash k$ (the Planck constant $h=2\pi \hslash$, where the $2\pi$ is injected into the $\hslash$, since physicists usually prefer to discuss the angular frequency $\omega=2\pi\nu$ and the wave vector $k=2\pi p$ rather than the frequency $\nu$ and the momentum $p$ ...

5

### Is there a way to create a flickering frequency to be dependent on speed of the person looking at it?

You can use a reflector with gaps. Then the light from a car will alternate between reflecting and not reflecting at a rate dependent on their velocity towards the reflector. Please excuse my crude diagram: As the car moves right to left, gaps in the reflector will cause it to appear to flash on an off.

4

### Standing Waves Energy transfer

Waves on strings combine linearly. This means that you can split up a string's motion into two (or more) superimposed waves. The two superimposed waves behave independently, as if the other one was not there. So if you have a standing wave set up on a string, and then you also introduce a travelling pulse, you get something like the following. (The arrows ...

4

### Boundary conditions on wave equation

The second condition is saying that there is no discontinuity in the slope of the rope at the junction. In other words, there is no "kink" in the rope. Imagine if this assumption were to fail in the following way: $$\frac{\partial D_1}{\partial x}(0,t) = -1, \qquad \frac{\partial D_2}{\partial x}(0,t) = 1$$ Then near the origin, the rope would look ...

3

### The second resonance of string?

The first resonant vibrational mode for a string clamped at both ends looks like: You should be able to deduce the wavelength from that diagram. The second mode looks like: Both of the images above are from http://www.clickandlearn.org/Physics/sph3u/Music/Music.htm and that site will spell it out in more detail for you. If your string length is ...

3

### Why do sound waves travel at the same speed moleculewise? (Same medium)

This is a very good question. I'm going to give you a more conceptual answer rather than the quick answer because I find this explanation helps my own students understand this better. First, consider yourself standing in a gymnasium with a thousand people in it. Not a lot of room is there? Naturally, you'd want some personal space, so you push at the people ...

2

### Lethality of sounds and extreme “loudness”

Your question in poorly defined because the concept of sound doesn't extend very nicely to non-atmospheric settings. Are gravity waves sounds? Are the pressure / shock waves in nebula? I don't think there is a unambiguously correct interpretation of sound for your question. Regarding lethal sound here on Earth, the answer depends on what you consider ...

2

### Lethality of sounds and extreme “loudness”

Well, we've classified a whole range of scales for the human hearing (which includes pure tone too). For lethal, we don't use how loud it should be, but instead - we say "how intense it should be" so that it can affect our ears. A quote from Wiki... Loudness, a subjective measure, is often confused with objective measures of sound strength such as sound ...

2

### Will changing amplitude change the frequency?

It's completely possible to change the amplitude (the difference between the maximun value of the wave and the minimun) without changing the frecuency. Think this in AC, where you can have signals with different voltage but the same frecuency. To illustrate it I'll show you this for a harmonic wave: $$x(t)=Acos(\omega t+\phi)$$ You can vary the amplitude ...

2

### How to determine the direction of medium's displacement vectors of a standing wave?

Any material between two nodes is displaced by the same direction. So the direction of B and C has to be the same as well as the direction of A and D due to symmetry. In addition, the direction of A must be the opposite of B since they are across from a node. Similarly the direction of C and D must be opposite. So the two possible configurations are ...

1

### How to determine the direction of medium's displacement vectors of a standing wave?

A standing wave is a wave that has nodes. The points of the wave go up and down in some places, and remain at zero at others (the nodes). The general form of a standing wave is a sine curve that remains at a fixed position, but its amplitude changes in time between $+A_0$ and $-A_0$. Specifially, there is a time where the wave form is completely flat. ...

1

### Lethality of sounds and extreme “loudness”

Sound as we know it is a disturbance of our atmosphere, transmitted as a wave to our ears - and yes, it can absolutely be lethal - shockwaves can hurt people very badly, as anyone who's been to the scene of a large explosion can attest. We typically measure "loudness" on a log scale of the pressure of the sound wave - I admit I'm unsure of how much pressure ...

1

### EM Waves Energy Loss

Each photon leaves its energy in the molecules of the screen. Destructive interference observed at the line x=1mm for example , means that the probability of finding a photon at x=1 is close to zero. Instead, the photon has very high probabiliy of depositing its energy at the construcive interference fringe.

1

### De Broglie wavelength, frequency and velocity - interpretation

Yes, the formulae $p=h/\lambda$ and $E=h\nu$ (the same equations as yours, reverted a bit) are universal – they hold not only for photons but for any particle. Also, these two equations aren't quite independent. Assuming special relativity, they both follow from the de Broglie form of the wave function which is pure phase:  \psi(x,t) = C\cdot \exp(2\pi i ...

1

### Why frequency and tension doesn't change in the two medium?

The tension in the two cords is the same because they are tied together. For example if the tension in the thick cord was higher than the thin cord the thick cord would shrink and the thin cord stretch until the tensions were equal again. The frequency has to be the same in both cords because the phase of the wave has to match at the junction between the ...

1

### Waveguides (in the ocean?)

You don't need a sharp discontinuity in the speed of sound to guide the waves. Remember that reflection does not occur right at the interface; rather, the wave always penetrates outside the waveguide to "see" what's going on there. A gradual increase in the speed of sound enforces the wave to reflect as well. Reflection occurs from above due to the ...

1

### Phase shift of 180 degrees on reflection from optically denser medium

The phase change happens because it is how waves behave. An additional link provides lecture notes. I know that u are not satisfied with this answer but you can compare this with mechanical waves in a string which gives better intuition by use of newtons laws.

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