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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1 answer
168 views

What happens if one end of an "ideal string" is vibrated at a frequency not equal to the harmonic modes?

Let's say we have a string of length $L$ for which the wave propagation speed is $c$. We consider vertical displacements $u(x, t)$ along the string on domain $x\in [0, L]$. The right end $x=L$ is ...
1 vote
1 answer
362 views

What does it mean when a function doesn't solve the wave equation?

The wave equation is $\frac{\partial^2 \chi}{\partial t^2} = c^2 \frac{\partial^2 \chi}{\partial x^2}$. I'll be understanding it in terms of sound. The wave equation is solved by many periodic ...
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1 answer
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Why do we only consider two points in Young’s double slit experiment?

Say we have two slits in the xy-plane. The x distance between them is d. Why do we, when calculating the resulting amplitude of the wave, consider only the phase difference of two secondary radial ...
6 votes
3 answers
482 views

Difference Between Particle Velocity and Wave Velocity in a Longitudinal Wave

In a longitudinal wave, both particle velocity and wave velocity occur in the same direction. It might seem intuitive to assume that the particle velocity should be identical to the wave velocity, ...
2 votes
3 answers
56 views

Why does air vibration produce a spherical wavefront and not a plane wavefront?

I want to know why the back-and-forth vibration of a material produces a spherical wavefront, for the produced sound wave, and not a plane wavefront.
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1 answer
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Acoustic wave incident to pipe wall

I would like to consider the sound incident to a water filled pipe wall. I think the pipe wall is typically considered as a rigid wall boundary, it means all the incident wave is reflected. Is this ...
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0 answers
21 views

Derivation of Lighthill Equation

I'm trying to understand the meaning of Lighthill equation. Based on my text book (sound and source of sound, Dowling and Williams 1983), it is derived from combining mass conservation and momentum ...
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2 answers
56 views

Is the Huygens' principle consistent for intersecting wavefronts?

When refraction takes place at the interface of two media, wavefronts can be extended to intersect as below: At point of intersection, light requires no time to travel between the wavefronts. However,...
1 vote
3 answers
7k views

Do electromagnetic waves produce sound?

Preamble: At 5 AM in the morning, if I sit quietly and listen attentively, I can hear many different noises, on a very high frequency. If I try to isolate the ones that are less noisy, I can clearly ...
3 votes
2 answers
112 views

Are EM waves telling us the probability of finding a photon?

I feel like I've been frequently presented with an interpretation of EM waves that goes something like this: Light is an oscillating electromagnetic field. Because changes in the electric field ...
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21 views

What should be minimum width of slit for interference pattern to occur in a single slit experiment? [duplicate]

When two slits produce an interference pattern, and one slit is closed, then the interference pattern disappears. Why? As one-slit interference is still possible. What should be the measurements of ...
0 votes
2 answers
80 views

Why intensity of unpolarised light is halved each time it passes through a polariser?

My doubt is Why is intensity halved each time an unpolarised light passes through a polaroid? My thought process is that if an unpolarised light passes through a polaroid, it will polarise the ...
1 vote
3 answers
135 views

What exactly is weird about waves propagating without a medium? What does it violate? [closed]

I can not seem to grasp, why exactly does light travelling through space without the need of any medium was baffling for the scientists of 1800's.
1 vote
1 answer
110 views

Can an electromagnetic wave be polarised in the direction it propagates?

Can the electric field vector of an EM wave oscillate in the propagation direction? In text books the polarisation is always orthogonal to the propagation direction. I'm wondering specifically ...
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1 answer
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Do even modes exist for e.g. pipes closed at one end?

This is really a question about terminology, The wavelength of a standing wave in a e.g. pipe closed at one end and open at the other is said to be $\frac{4L}{n}$, where $L$ is its length and $n$ is ...
4 votes
1 answer
604 views

Why is phase velocity not a vector and what is the conceptual meaning of "phase slowness"?

I was reading Principles of Optics by Born and Wolf; there they introduced the concept of phase velocity as follows: [...] If we denote $\mathbf q$ the unit vector in the direction of $\mathrm d\...
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1 answer
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Applications of Schrodinger's to dark solitons [closed]

The Schrodinger equation (SE) admits dark solitons as particular solutions. The SE and the The Korteweg-de Vries (KdV) equations can be used to model them. Questions: What are the applications of ...
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1 answer
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Modulation Index of an AM wave

I have been given a question as RMS value of an RF(Radio Frequency) voltage after amplitude modulation to a depth of $50$% by a sinusoidal voltage is $80$ Volts. Calculate the RMS value of modulated ...
0 votes
1 answer
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Direction of propogation of a EMW [closed]

If let say i have an EMW given by- (Note the difference between $k$ and K) B(x,y,z)=$B_0sin[(x+y)\frac{K}{√2}+wt]\hat k$ i got confused in 2 different outcomes when i wanted to find out the direction ...
2 votes
1 answer
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What is the physical explanation as to why the kinematic boundary condition must hold at the free surface of a wave?

The kinematic boundary condition at the surface of a water wave is given as: "a particle on the free surface remains there always". This is then written as the material derivative of the free surface ...
0 votes
1 answer
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How to add a transverse force to a point on a simple wave equation?

Background Imagine simple wave equation, say of a string fixed at both ends, in this form: $$ρ y_{tt} = \frac{T}{A}y_{xx}$$ Or with an added velocity damping term like: $$ρ y_{tt} = \frac{T}{A}y_{xx} -...
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35 views

How much diffraction happens when the size of the gap is less than the wavelength of the water wave?

If a parallel water wave (water wave with straight wavefronts) is incident on a gap, diffraction happens. However, how much diffraction happens depends on the relative size of the gap to the ...
3 votes
0 answers
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Why do water surfaces have persistent "average wavelength" patches?

Have you ever stood above a river or lake and noticed that the surface has visible "patches"? It looks like the surface has different average wavelengths in some areas, leading to the light ...
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2 answers
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Where is the compression for this sound wave?

I was given this question for high schoolers - which I'm a bit embarrassed I can't answer. Fig. 1 shows the equilibrium positions of 14 equally spaced air molecules, labelled 1 to 14, along a line AB....
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4 answers
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Why does the sine wave graph gets inverted when we put $t=0$ in $A\sin(wt-kx)$ and what does it signify?

I was studying standing waves today and the teacher took a string attached to a fixed end then waves are produced (of equation $Asin(\omega t-kx)$ )on the string. and then he told us that the ...
2 votes
2 answers
157 views

Proving a given vector to be a four-vector

How to prove that $$k=(\frac{\omega}{c} , \vec{k})$$ is a four-vector? Where $\omega$ is the frequency and $\vec{k}$ is a wave vector.
2 votes
1 answer
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Differences in the direction of energy and wave propagation in anisotropic media

I have been studying how light behaves in dielectric and conductive media, and now I was about to study it in anisotropic media. The introduction to the subject is: "From Maxwell's equations it ...
0 votes
0 answers
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Printing 2d fresnel zone plate

Recently my teacher asked me to create a Fresnel zone plate (Fresnel lens to be accurate). I wanted to print it using a laser printer with a resolution of 1200*1200 dpi. I know that i should block ...
1 vote
1 answer
2k views

How does a longer wavelength penetrate deeper with Rayleigh waves?

I'm struggling slightly to understand this idea. I've slowly been building up an explanation, so at this stage it might be just some confirmation I'm looking for, but also some guidance if I'm off ...
1 vote
0 answers
29 views

Why do light bulbs don't produce coherent waves? [duplicate]

My textbook says that light ways produced by light bulbs are not coherent but it doesn't describe the reason. I was wondering how could two waves not be coherent regardless of the source they are ...
3 votes
2 answers
785 views

What determines how much power goes into each diffraction order?

Imagine a grating with infinite number of slits, and the spacing D between slits is larger than the wavelength so that there are high order diffractions. In each of the diffraction directions the ...
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1 answer
55 views

Where does the diffraction equations work and why?

w = fringe spacing $\lambda$ = wavelength D = horizontal distance between slits and screen s = slit spacing d = slit spacing $\theta$ = angle between central maximum and the ray at the nth maximum $\...
0 votes
0 answers
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Effective aperture as a function of Azimuth and Elevation Angle

Wikipedia says The effective aperture of an antenna is given by $$A_{\mathrm{e}}(\theta, \phi)=\eta A \cos \theta \cos \phi ,$$ where, $(\theta, \phi)$ are the azimuth and elevation angles relative to ...
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2 answers
141 views

Graphical interpretation of complex electric fields

Before anything, I have read similar questions here but still something doesn't click perfectly, so I'll try to describe so. My doubt is essentially that I have always thought $\vec E$ in the context ...
11 votes
6 answers
56k views

What is the difference between a pulse and a wave?

I wanted to ask what is the difference between a pulse and a wave? According to their definitions, they are almost the same. In the websites I looked at, the difference between them was described by ...
0 votes
2 answers
109 views

Wave dispersion relation as a differntial equation

I have seen the following dispersion relation as a differential equation: $$ \frac{ \partial \mathbf{k} }{ \partial t } + \left( \mathbf{V}_{g} \cdot \nabla \right) \mathbf{k} = 0 \tag{1} $$ in the ...
2 votes
0 answers
137 views

Holographic principle and Huygens principle

I have recently read about the information paradox of black holes and how it lead eventually to the formulation of the holographic principle. Quoting Wikipedia: "the description of a volume of ...
21 votes
1 answer
4k views

Slinky reverb: the origin of the iconic Star Wars blaster sound

This is a fun problem that I came across recently, which I'm posting here for your delectation. We all love a good slinky: they can be used for all sorts of fun demos in physics. One example is the "...
6 votes
2 answers
162 views

Is each INDIVIDUAL photon a PHYSICAL wave? [closed]

Sigh. So I've scoured the internet for many many hours, on many many occasions... aaand, yeah. Is light: just ONE photon which acts like a physical wave as those seen in classical mechanics (if so, ...
0 votes
1 answer
221 views

What is the function of the amplitude of a plucked string depending on where it is plucked?

Here is the image of the situation: Where L is the lenght of the string, A, the amplitude and p is a fraction of the string's lenght (where it is plucked = pL). So, here it is, How do I find an ...
0 votes
1 answer
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Multiple same maximum amplitudes of a pendulum

In the pic, you see a sphere freely hanging from the ceiling by a string. According to the simple definition of a pendulum, it seems to be okay to call it a pendulum. An instantaneous force, shown by ...
-3 votes
2 answers
113 views

How many 'wavelengths of length' can a wave have? [closed]

Since a wavelength, $\lambda$, is the length of a entire cycle How many $\lambda$ (complete cycles) can a composed wave have? I mean, for $n \lambda$, how big can $n$ be? And what does it mean, ...
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0 answers
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Is there a ponderomotive force due to a traveling wave?

Imagine a traveling, EM wave passing through some plasma. The waveform is given by the vector potential: $A = (0, -\frac{cE_0}{\omega_0}\cos\eta, 0)$ where $\eta = \omega_0t-k_0x$ is the phase. From ...
1 vote
1 answer
260 views

Transverse displacement profile of the string using Fourier series

A string of length $L$ fixed at $x=0$ and $x=L$ and released at time time $t=0$, the transverse displacement at a position $x$ along the string is given by: $y(x,0)=Ax(L-x)$ Assuming that the string ...
3 votes
3 answers
295 views

Why is the speed of sound wave in a gas always lesser than the r.m.s. speed at the same temperature?

My book says that the speed of sound wave in a gas is always lesser than the r.m.s. speed of the gas at the same temperature. I understand that speed of sound is given by: $$v_s=\sqrt{\frac{\gamma RT}{...
0 votes
1 answer
30 views

Frequency of light wave doesn't change with media

I have been trying to derive snell's law in the case of a monochromatic harmonic plane wave at a planar interface. By the same derivation I also wanted to derive how the frequency of the wave does not ...
0 votes
1 answer
196 views

Convex Lens forming plane wavefront

As we can see in the above diagram, as we move along the symmetric axis of the convex lens, D varies. Therefore the rays coming out of the point source hitting different points of the lens surface ...
1 vote
4 answers
98k views

How does increasing the tension on a string affect the density?

Consider a string under tension, for example, a string on a guitar. When a guitar string is plucked, it vibrates at a certain frequency. When the tension on the string is increased by twisting the ...
3 votes
2 answers
529 views

Poynting vector of a scalar wave

Consider a scalar wave (of sound for example) of the form $\psi(\vec{r},t)$. Which is its Poynting vector? I mean, the vector that takes account for the energy flux in the same sense as for the case ...
0 votes
2 answers
309 views

Why is this question indicating a Fraunhofer Region rather than Fresnel Region?

I was thinking that because the source plane is infinitely large would be analogous to having the source plane being infinitely close, which would imply a Fresnel region rather than Fraunhofer. ...