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Questions tagged [waves]

Waves are disturbances that propagate through space and time. Classically, they travelled through a medium, disturbing the particles but not changing their mean position. Electromagnetic waves/particle-waves need no medium; they are disturbances in their respective fields.

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536 votes
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Can I compute the mass of a coin based on the sound of its fall?

The other day, I bumped my bookshelf and a coin fell down. This gave me an idea. Is it possible to compute the mass of a coin, based on the sound emitted when it falls? I think that there should be a ...
Vinicius L. Beserra's user avatar
161 votes
4 answers
15k views

Why are the harmonics of a piano tone not multiples of the base frequency?

I was trying to figure out which piano keys were being played in an audio recording using spectral analysis, and I noticed that the harmonics are not integer multiple of the base note. What is the ...
Szabolcs's user avatar
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138 votes
15 answers
101k views

What happens to the energy when waves perfectly cancel each other?

What happens to the energy when waves completely cancel each other out via destructive interference? It seems like the energy just disappears, but that would violate the law of energy conservation. My ...
aortizmena's user avatar
  • 1,556
88 votes
4 answers
8k views

Why does rainwater form moving waves on the ground? Is there a name for this effect? [duplicate]

A while ago it was raining and I noticed that, on sloped pavement, water was flowing in very regular consistent periodic waves, as you see below. However, I realized I had no idea why this should be ...
user541686's user avatar
  • 4,201
87 votes
4 answers
6k views

Why do travelling waves continue after amplitude sum = 0?

My professor asked an interesting question at the end of the last class, but I can't figure out the answer. The question is this (recalled from memory): There are two travelling wave pulses moving in ...
Dylan's user avatar
  • 1,051
82 votes
8 answers
19k views

Why does fire make very little sound?

Sound is air particles vibrating (thus hitting each other to make longitudinal waves) and heat is the vibration of air molecules. Because we can only assume that heat made from fire is a higher ...
yolo's user avatar
  • 2,650
78 votes
5 answers
8k views

Why can we distinguish different pitches in a chord but not different hues of light?

In music, when two or more pitches are played together at the same time, they form a chord. If each pitch has a corresponding wave frequency (a pure, or fundamental, tone), the pitches played together ...
chharvey's user avatar
  • 868
77 votes
3 answers
14k views

Why doesn't the motion of a car affect the frequency of radio stations?

When we go in a car and tune to an FM radio station, why doesn't our motion disturb the frequency? Like the Doppler effect?
DARU SRINIVAS's user avatar
75 votes
4 answers
16k views

If water is incompressible, how can sound propagate underwater?

Since sound travels as longitudinal waves, sound waves should only be able to propagate in a medium through compressions and rarefactions. However, water, as a liquid, is generally treated as an ...
Omnitragedy's user avatar
73 votes
6 answers
12k views

Why don't choir voices destructively interfere so that we can't hear them?

Sound is propagated by waves. Waves can interfere. Suppose there are two tenors standing next to each other and each singing a continuous middle-C. Will it be the case that some people in the ...
chasly - supports Monica's user avatar
65 votes
7 answers
16k views

How is the Schroedinger equation a wave equation?

Wave equations take the form: $$\frac{ \partial^2 f} {\partial t^2} = c^2 \nabla ^2f$$ But the Schroedinger equation takes the form: $$i \hbar \frac{ \partial f} {\partial t} = - \frac{\hbar ^2}{...
user28823's user avatar
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64 votes
4 answers
8k views

Why does the sun have to be nearly fully covered to notice any darkening in an eclipse?

I was looking at eclipse footage and I noticed that it doesn't get any noticeably darker until the very end when it suddenly all the light is gone. As the moon blocks out the Sun, I would expect that ...
Giulio Crisanti's user avatar
62 votes
12 answers
25k views

Are there pure sine waves in nature or are they a mathematical construct that helps us understand more complex phenomena?

I've studied a bit of frequency analysis with FFT and optimal phase binning and was taught that we can represent any composite waveform as the sum of its component frequencies. I understand the maths ...
Schizomorph's user avatar
61 votes
4 answers
32k views

Why do prisms work (why is refraction frequency dependent)?

It is well known that a prism can "split light" by separating different frequencies of light: Many sources state that the reason this happens is that the index of refraction is different for ...
Brandon Enright's user avatar
60 votes
2 answers
4k views

Why aren't the lengths of the bars on a toy glockenspiel proportional to the wavelengths?

As you might already know, frequency of musical notes is arranged in a such a way that if, for example, an A note has frequency of $x$, another A note which is placed one octave higher would produce ...
Moctava Farzán's user avatar
60 votes
6 answers
82k views

Phase shift of 180 degrees of transversal wave on reflection from denser medium

Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave, when reflected from an optically denser medium? I tried searching for ...
user avatar
55 votes
9 answers
110k views

Why does wavelength affect diffraction?

I have seen many questions of this type but I could nowhere find the answer to "why". I know this is a phenomenon which has been seen and discovered and we know it happens and how it happens. But my ...
rahulgarg12342's user avatar
51 votes
9 answers
16k views

Why exactly does diffraction occur?

Why do waves that were traveling in a straight direction change direction when passing through an opening? I thought that the waves (red arrow) when colliding with the wall bounce in the opposite ...
jony alton's user avatar
49 votes
7 answers
12k views

Why don't two musical instruments sometimes generate destructive interference?

I'm an electrical engineer, and I understand wave propagation, interference patterns, and so on. But I'm missing something basic, so perhaps my understanding isn't as good as I believe. I'll show my ...
bitsmack's user avatar
  • 718
49 votes
8 answers
23k views

Amplitude of an electromagnetic wave containing a single photon

Given a light pulse in vacuum containing a single photon with an energy $E=h\nu$, what is the peak value of the electric / magnetic field?
Andrey S's user avatar
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49 votes
6 answers
4k views

The speed of sound is a single value yet the speed of atoms is distributed over many values. Does the sound wave front smear out?

Sound waves travel with constant speed, but air molecules that transfer action move with different speeds than the ones described by Maxwell distribution. Why does the sound wave not smear out and ...
Vashu's user avatar
  • 629
46 votes
10 answers
9k views

Why do string instruments need hollow bodies?

My textbook states, 'The sound box has a large area, it sets a large volume of air into vibration, the frequency of which is same as that of the string. So due to resonance a loud sound is produced.' ...
Hayden Soares's user avatar
45 votes
7 answers
6k views

Why is it so easy to create audible sound?

Context Why is it so easy to create audible sounds in life with basically anything? Putting your cup of coffee on a table comes with a sound Turning a page of your book comes with a sound Even ...
user929304's user avatar
  • 4,685
45 votes
4 answers
6k views

Why is Huygens' principle only valid in an odd number of spatial dimensions?

Apparently Huygens' principle is only valid in an odd number of spatial dimensions: https://mathoverflow.net/a/5396/21349 Huygen's principle in curved spacetimes Why is this? [EDIT] This is ...
user avatar
44 votes
9 answers
15k views

If a thousand people whisper inaudibly, will the resulting sound be audible?

If a thousand people whisper inaudibly, will the resulting sound be audible? (...assuming they are whispering together.) I believe the answer is "yes" because the amplitudes would simply add and thus ...
SAH's user avatar
  • 553
43 votes
5 answers
121k views

Why does medium not affect the frequency of sound?

I read in various places that frequency does not change with medium. Instead, wavelength changes in different mediums due to a change in speed. I understand why speed changes with medium, but I'm not ...
carbenoid's user avatar
  • 553
41 votes
2 answers
4k views

Isn't D'Alembert's wave equation enough to see that Galilean transformations are wrong?

The D'Alembert equation for mechanical waves was written in 1750: $$\frac{\partial^2u}{\partial x^2}=\dfrac{1}{v^2}\dfrac{\partial^2u}{\partial t^2}$$ (in 1D, $v$ being the propagation speed of the ...
Tropilio's user avatar
  • 650
41 votes
4 answers
6k views

Validity of naively computing the de Broglie wavelength of a macroscopic object

Many introductory quantum mechanics textbooks include simple exercises on computing the de Broglie wavelength of macroscopic objects, often contrasting the results with that of a proton, etc. For ...
Mark Allen's user avatar
39 votes
4 answers
8k views

Why can a flugelhorn easily play its fundamental frequency, when a trumpet can't?

Some Background I became fascinated with how the overtone series, or harmonics, relates to how brass instruments function. Most trumpet players (or brass, really) should notice that as they play ...
fyrepenguin's user avatar
39 votes
2 answers
10k views

Are gravitational waves longitudinal or transverse?

Waves are generally classified as either transverse or longitudinal depending on the they way the propagated quantity is oriented with respect to the direction of propagation. Then what is a ...
Ignacio's user avatar
  • 645
38 votes
2 answers
7k views

Why does ice make such peculiar sounds? [duplicate]

I've come across a couple of videos where some interesting sounds are produced using ice. (Click on the images to see the video.) Here, they drop a block of ice into a deep crevice and as the block ...
AlphaLife's user avatar
  • 12.5k
37 votes
5 answers
6k views

What do we see while watching light? Waves or particles?

I'm trying to understand quantum physics. I'm pretty familiar with it but I can't decide what counts as observing to cause particle behave (at least when it's about lights). So the question is what do ...
martintrapp's user avatar
37 votes
3 answers
6k views

Is it possible to estimate the speed of a passing vehicle using a musical ear and the doppler effect?

I've found a number of questions that concern the Doppler effect, but none that seem to address my question. I have a background in music. People with a musical ear can generally tell the ratio ...
M_M's user avatar
  • 481
37 votes
3 answers
11k views

Do photons occupy space?

Total noob here. I realize that photons do not have a mass. However, they must somehow occupy space, as I've read that light waves can collide with one another. Do photons occupy space? and if so, ...
LanceLafontaine's user avatar
36 votes
5 answers
5k views

Why does the pet's water bowl overflow?

So when i give the pet fresh water in a stainless steel bowl that i place on a mat according to the attached picture, from $t=0$ the bowl is at rest, the water normally oscillates in the bowl like a ...
user721108's user avatar
34 votes
8 answers
17k views

Since cables carry electricity moving at the speed of light, why aren't computer networks much faster?

Why can't cables used for computer networking transfer data really fast, say at the speed of light? I ask this because electricity travels at the speed of light. Take Ethernet cables for example, I ...
Celeritas's user avatar
  • 451
34 votes
8 answers
45k views

Why do tsunami waves begin with the water flowing away from shore?

A sign of a tsunami is that the water rushes away from the shore, then comes back to higher levels. It seems that waves should be both + and - polarized and that some tsunamis should go in the ...
Carl Brannen's user avatar
  • 12.8k
34 votes
2 answers
12k views

Pouring oil on choppy water to calm it , does it work and if so how?

Near where I live, local fishermen often bring cans of castor oil with them, to calm the water around their boats, if they feel bad weather is due. They claim this method of sea calming works, (...
user avatar
33 votes
5 answers
6k views

If two sound waves that are different frequencies create beats that occur several hundred times per second, can you hear this effect as its own tone?

If you have multiple waves of different frequencies, the interference from the different waves cause "beats". (Animation from https://en.wikipedia.org/wiki/Group_velocity) Let's say that a green dot ...
Daniel M.'s user avatar
  • 445
32 votes
7 answers
11k views

Why do higher harmonics have a lower amplitude than the fundamental frequency?

When we pluck a string, it vibrates in all possible modes of vibrations. The lowest frequency possible is the fundamental frequency and it is the most significant part of sound. But why do the ...
Rahul R's user avatar
  • 1,027
32 votes
4 answers
11k views

First-order wave equation: Why is its presence not common?

The (one-dimensional) wave equation is the second-order linear partial differential equation $$\frac{\partial^2 f}{\partial x^2}=\frac{1}{v^2}\frac{\partial^2 f}{\partial t^2}\tag{second order PDE}$$ ...
BMS's user avatar
  • 11.5k
32 votes
3 answers
41k views

Theory behind patterns formed on Chladni plates?

In this video of vibrating Chladni plates we can see small sand particles align themselves into different interesting patterns (also shown in the image below) which correspond to some particular ...
Beni Bogosel's user avatar
31 votes
11 answers
24k views

Why no longitudinal electromagnetic waves?

According to wikipedia and other sources, there are no longitudinal electromagnetic waves in free space. I'm wondering why not. Consider an oscillating charged particle as a source of EM waves. Say ...
Hugh Allen's user avatar
  • 1,505
31 votes
4 answers
4k views

How can transverse waves on a string carry longitudinal momentum?

In general, if a wave carries energy density $u$ with velocity $v$, it also carries momentum density $u/v$. I've seen this explicitly shown for electromagnetic waves and (longitudinal) sound waves. ...
knzhou's user avatar
  • 103k
31 votes
1 answer
19k views

How can shock waves travel faster than sound?

A shock wave can be caused by the disturbance of air by an airplane. When it propagates, shouldn't the mechanism be the same as that of a longitudinal sound wave? Why can a shock travel faster than ...
Kelvin S's user avatar
  • 1,135
30 votes
10 answers
8k views

If light propagates like waves, why can't I see around corners?

I know two different descriptions of how light propagates in space; (1) like particles traveling and reflecting in straight lines. And (2) like waves spreading and interfering in space. And that both ...
erik m's user avatar
  • 1,153
30 votes
7 answers
9k views

Why does blowing a whistle in someone's ear damage it more than blowing directly in their ear? Won't the whistle reduce overall energy?

If I blow really hard from a whistle near someone's ear, it'll hurt a lot. But if I blow directly at a person's ear, it won't hurt nearly as much. But shouldn't the whistle (or any other obstruction) ...
chausies's user avatar
  • 1,090
30 votes
4 answers
8k views

Speed of sound at temperatures below 0 °C

How can the speed of sound be calculated for temperatures below 0 °C (down to -40 °C)? Does the calculation $v=331\ \frac{m}{s} + 0.6 \frac{m}{s°C} \times T$ still hold (where T's unit is °...
user40343's user avatar
  • 269
29 votes
6 answers
6k views

Do all waves of any kind satisfy the principle of superposition?

Is it an inherent portion of defining something as a wave? Say if I had something that was modeled as a wave. When this thing encounters something else, will it obey the principle of superposition. ...
JobHunter69's user avatar
  • 1,325
28 votes
4 answers
7k views

Why is the speed of oceanic waves not a constant like sound?

I cant understand this, according to what I read here. The speed of a wave depends on its wavelength and its depth, through the relation $$ v=\sqrt{\frac{g\lambda}{2\pi}\tanh\left(2\pi \frac d\lambda\...
papajo's user avatar
  • 401

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