Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: EvaH on 28/04/2020 14:04:45
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Wally wants to know:
How similar is sound and light? I know both are measured in wavelengths: could light be a high frequency sound, or sound a low frequency of light?
What do you think?
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No.
Sound is longitudinal compression waves of a medium. Light is electromagnetic radiation with no requirement for a medium of transmission.
Sound does not travel through a vacuum. Light travels fastest, and with least attenuation, through a vacuum.
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A couple of other points.
Light is a transverse wave, not a longitudinal one like sound.
The speed of a sound wave relative to you depends on your velocity with respect to the medium, while the speed of light in a vacuum, relative to you ( as measured by you) is always the same value (c).
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It made me wonder, also made me think of that catchy warning saying that 'No one will hear you scream in a vacuum'. A scream is expending energy, isn't it? So where does it go, in a vacuum?
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It doesn't "go"; there's nowhere for it to go to.
So, if you scream in a space suit, the sound inside the helmet is slightly louder because none gets out.
And it echoes round until it is absorbed- mainly by your head- and warms it up a bit.
It's less clear cut if you are not in a space suit- but, in that case, it wouldn't' matter much even if someone did hear you.
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Yeah BC, that's the one I was thinking of, no space suit.
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But it is molecules particles 'vibrating', right? Gaining energy by me shouting, exciting them?
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spelling
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Yeah BC, that's the one I was thinking of, no space suit.
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But it is molecules particles 'vibrating', right? Gaining energy by me shouting, exiting them?
Imagine you had a lung full of air, You scream. The air is forced past your vocal chords which vibrate, modulating the air leaving your mouth and producing alternating waves of air compressions. These compressions are further propagated by the air outside your body.
Now, what happens if there were no air outside of your body.
The air inside your lungs is made up of molecules which are themselves traveling at something like 340 m/sec. The air molecules are close enough together that they can't travel very far before bumping into another molecule (the mean free path). This distance is very small compared to the wavelength of a sound wave. which is why air at standard pressure can carry sound waves for long distances.
So when the air which has been modulated by your vocal chords leaves your mouth, there are no air molecules out there to "contain" them. Instead of bumping into other molecules and creating sound waves, they scatter and spread out a 340 m/sec.
So the "energy" from the sound is rapidly being spread out thinner and thinner over this expanding volume.
Not only that, but the mean free path of the molecules increase; they individually travel further and further before encountering another molecule. Thus the mean free path becomes long compared to the wavelength of the "sound", eventually surpassing it.
The upshot is that the "sound energy" becomes more and more just a matter of the average kinetic energy of the molecules which is randomly distributed and no longer takes the form of discernible sound waves.
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But it is molecules particles 'vibrating', right?
No.
The vibrational frequencies of molecules are roughly in the MHz to GHz range.
Sound is a variation in the pressure of molecules; a measure of how closely packed they are.