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Author Topic: why can sound waves go through some solids that light waves can't?  (Read 5164 times)

Offline annie123

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probably a simple answer I know, but I don't know it . . .Why can sound waves pass through more solids than light waves?
" Wherefore one can inside a wall see naught,
     Yet catch the voices from beyond the same." Lucretius  De Rerum Natura
« Last Edit: 14/02/2013 23:48:04 by annie123 »


 

Offline Pmb

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Any opaque sold will propagate sound waves but not light waves. This is not so for other ranges of the EM spectrum such as the X-ray spectrum,
 

Offline evan_au

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Every uniform solid (and, to a lesser extent, liquids and gases) will propagate sound, because the movement of one atom (ie sound) will be transferred to adjacent atoms by their touching electric fields. This allows sound to propagate fairly easily through the body of the solid, until it eventually turns into heat in the solid.

However, light is an electromagnetic wave. Light will interact with any electrons in a uniform solid having available energy levels they can jump into, absorbing the photon. This energy will be re-emitted a random time later, in a random direction, and usually as several photons of lower energy than the original photon; this energy eventually turns into heat in the solid. The wavelengths of light that can be absorbed is different for each chemical, so any material made of many different chemicals is likely to absorb every wavelength of light.

Some elements like metals with partially-filled shells have many electrons, with many energy levels they can jump into, and so can absorb light of many different wavelengths and reradiate it at many longer wavelengths; a small fraction of metal atoms in a material can make it opaque.

Thin sheets of silicon dioxide used in window glass do not strongly absorb energy in visible wavelengths (but it does absorb in the ultraviolet and infra-red regions of the electromagnetic spectrum).

Note that non-uniform materials which differ greatly in their density or refractive index will scatter sound and/or light respectively, without actually absorbing it.
 

Offline yor_on

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I agree on all but that the 'new' light will be 'randomly ejected', as far as I get it it will have a direction inside the substance consistent with the 'path' originally created. Or maybe you mean that it can 'bend' due to the material it 'propagate' in?
 

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as far as i have thought, when photon hits an atom, the atom absorbs the energy and immediately releases producing light of specific wavelength. No matter from which direction photon hits, atom discharges in all directions ultimately dividing energy.

In case of opaque objects, suppose first layer of atoms absorbs light and and discharges it, so the second layer of atoms will get about half the energy. Like this a layer would come which will not get enough energy to pass and hence the viewer standing on the opposite side will not be able to see through the object.

For instance, try to look through your finger(actually there is no need to do it), it's opaque. But when you put your finger on torch, finger becomes red, meaning it is now passing the light. I think this clears what i am trying to say.

So wall doesn't pass light as light falling on it is not enough strong.
Reason for sound is already given by somebody.
 

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now question arises about transparent materials. Simple explanation about it is, there is much more gap between two atoms of transparent materials.

 I am just trying to give reason for it so if anybody knows real reason pls tell that.
 

Offline cheryl j

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What happens with radiowaves?
 

Offline Pmb

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What happens with radiowaves?
Something similar. All material has an opacity in a given range of frequencies. Radio waves can go through black paper but light waves can't
 

Offline yor_on

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If that was true Scientist a laser light 'propagating' through water or glass would have no directionality.
 

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If that was true Scientist a laser light 'propagating' through water or glass would have no directionality.
I think after passing through glass or water, laser beam does not go in straight line, it goes in somewhat divergent manner(i.e. after passing through glass or water the 'dot' of laser becomes larger). (i'm not sure about it, and can't do the experiment just now. I'm only posting what I think.)

Laser light has something higher (lets say energy or ability to go as straight as possible) than ordinary light. When the laser beam passes through glass, due to lower optical density most of the light goes straight. But some of those hit atoms and and as I've explained before this light is absorbed and emitted in all directions. And we get the dot bigger.
 

Offline evan_au

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Quote
light... will have a direction inside the substance consistent with the 'path' originally created

This is true in transparent materials - the photon is re-emitted with the same energy, in the same direction, within nanoseconds 10-18s (see calculation in subsequent post, below).

However, in materials which absorb the photon, an electron will be raised to a higher energy level.
Some time later, the electron will fall back to a lower energy level, often dissipating some or all of the photon energy as heat, and sometimes re-emitting a photon, sometimes in a different direction. A material which re-emits even 10% of the incident photons is considered fluorescent.
The electron does not necessarily fall back to the original energy level, but may cascade as a series of transitions of lower energy, emitting photons of lower energy in different directions. http://en.wikipedia.org/wiki/Fluorescence
This time can be quite long in a phosphorescent material (ie seconds or minutes): http://en.wikipedia.org/wiki/Phosphorescence

A somewhat different effect can occur in a laser medium, where most electrons are "pumped" to a high energy level by  photons or electric currents, and then decay to a slightly lower energy level, which is metastable. When a photon of the right frequency arrives, it triggers the electron to drop to the low energy level, emitting a photon which has the same frequency and phase as the incident photon. The laser photon has a slightly lower energy than the "pump" photon.  http://en.wikipedia.org/wiki/Laser#Stimulated_emission
« Last Edit: 17/02/2013 20:18:54 by evan_au »
 

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what happens in mirrors?? How angle of incidence is equal to angle of reflection?? And why it is in single plane??
 

Offline CliffordK

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now question arises about transparent materials. Simple explanation about it is, there is much more gap between two atoms of transparent materials.
I don't believe that is true.

For example, Graphite is opaque in the visible light spectrum.
Diamond, made out of the same carbon, but at a higher density is transparent under visible light.

Different substances may be transparent to some parts of the light spectrum, and opaque to others.  Silicon is generally opaque under visible light, and transparent under the longer wavelenth IR light.

Consider colored glasses, absorbing a single wavelength of light.
 

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it was just an attempt to explain it.
Optical density is different from the material density. Kerosene is rarer than water but its optical density is more than water.
 

Offline evan_au

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A correction to a previous post, after doing some calculations:
The delay of light due to passing through a unit cell of diamond is about 7x10-19s.

Calculation:
  • Speed of light in a vacuum: 3x108 m/s
  • Refractive index of diamond: 2.4 in visible light (quite high, as you would expect from a gemstone)
  • Speed of light in diamond: (1)/(2)=1.2x108 m/s
  • Bond length of diamond: 1.54x10-10m
  • Time for light to propagate this distance in diamond: 1.2x10-18s
  • Time for light to propagate this distance in a vacuum: 5.1x10-19s
  • Delay of light due to passing through a carbon atom (in diamond): 7.2x10-19 s

PS: A diagram comparing the energy decay paths for heat-producing, fluorescent and phosphorescent interactions with light is shown here: http://en.wikipedia.org/wiki/File:Electronic_Processes_Involving_Light.png
 
 

Offline yor_on

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Not sure about this, I would say that there is a momentum, and with that a directionality.
 

Offline evan_au

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Quote
Scientist: what happens in mirrors?? How angle of incidence is equal to angle of reflection?? And why it is in single plane??

See recent post on: http://www.thenakedscientists.com/forum/index.php?topic=46992.msg405400#msg405400
« Last Edit: 19/02/2013 16:32:30 by evan_au »
 

Offline yor_on

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It's quite tricky. If we involve 'phonons' it becomes a real headache to discuss any momentum to sound/vibrations in a lattice. If 'phonons' are considered waves they must have a momentum though, as I understands it, and with that a directionality.

This one http://arpes.stanford.edu/research_arpesoverview.html is interesting when it comes to how solids react on photons. and there the momentum imply a direction, as I read it.
 

Offline yor_on

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Annie, what we can say, you and me all :) is that sound indeed are transmitted through a solid, but also that a light quanta disappear, transforming its momentum and energy into heat (as a first guess). Then we have to look at how sound is transmitted. Molecules and atoms inside a lattice set into vibration seems a close approximation to what sound is, to me. And there we find that those atoms and molecules more or less oscillate, meaning that behave like springs undulating around some center. They do not move themselves, each one of them stays in place, which is rather obvious as we otherwise would have solids becoming 'plastic/melting' with sound :) so mathematically those oscillations take themselves out, giving no net directionality (force) from the 'sound/vibrations' induced into the material.

And that is weird as we all know that sound do transmit through solids. So what one can do then is to lift some higher order of 'motion' not involving deep studying each unique particle but instead look at how the pattern of all those particles behave from a distance. Then they start to move in a direction, meaning that the overall pattern indeed seems to transfer 'something' in a direction, all as I get it. What transfers this is named 'phonons' and depending on how you look at that it's either a figment of ones fantasy or possibly some sort of bosons (like the Higgs, or photons). If it is a boson it might be seen as a wave, and if it is a wave it should have a momentum, as I think then, and that should present it with a directionality.

When it comes to a photon, it has a momentum and a energy, which is why I expect it to create a 'path', or preferred direction inside a solid too. But it is also so that the energy states inside a atom is quantized, so when you transfer energy in form of a photon the orbitals 'light/jumps up' as a whole making it hard to define how this directionality comes to express itself, but there must be a preferred direction inside a solid too. As Evan points out it doesn't seem as if new photons created inside the material always take that path but there should be a preference for it at least, or else I don't know a thing about what a momentum is :)
 

Offline yor_on

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Just a weird thought.

A different way to define a momentum might be that we define it such as what we see as its directionality is a property having another degree of freedom, what we call mass and velocity, or the inertial energy of that object expressed through adding motion. Not that I see how that should be defined logically but using it we could assume that each interaction is concentric to its expression, with what we call momentum being the directionality we find through that 'degree of freedom'. And then it comes down to what a 'speed' is as it seems to me, or what 'motion' in general really means?

Which indeed seems a weird property, considering uniform motion and accelerations.
(As well as the observer dependencies naturally, you can't exclude them other than strictly locally)
« Last Edit: 20/02/2013 08:13:22 by yor_on »
 

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