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Author Topic: Does air pressure slow light down?  (Read 951 times)

Offline Thebox

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Does air pressure slow light down?
« on: 05/07/2016 12:15:08 »
When air is dense (low pressure)  , does this slow light down compared to high pressure air (thin air)?


 

Offline dlorde

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Re: Does air pressure slow light down?
« Reply #1 on: 05/07/2016 13:16:05 »
When air is dense (low pressure)  , does this slow light down compared to high pressure air (thin air)?
Are you sure you have your pressures the right way around? in my experience, air pressure is highest where it is densest (e.g. ground level), and lowest where it is thinnest (e.g. top of Everest). Light will be slower in the more dense air.
 

Offline Colin2B

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Re: Does air pressure slow light down?
« Reply #2 on: 05/07/2016 13:25:03 »
When air is dense (low pressure)  , does this slow light down compared to high pressure air (thin air)?
Are you sure you have your pressures the right way around? in my experience, air pressure is highest where it is densest (e.g. ground level), and lowest where it is thinnest (e.g. top of Everest). Light will be slower in the more dense air.
That's correct. Also cold air is denser and hot air less so which is why we get mirage effects close to a hot roadway.

Good one Box, pretty close.
 

Offline Thebox

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Re: Does air pressure slow light down?
« Reply #3 on: 05/07/2016 13:30:19 »
When air is dense (low pressure)  , does this slow light down compared to high pressure air (thin air)?
Are you sure you have your pressures the right way around? in my experience, air pressure is highest where it is densest (e.g. ground level), and lowest where it is thinnest (e.g. top of Everest). Light will be slower in the more dense air.

I probably do have them the wrong way around, but thanks you did answer my question.

So if I was to send a satellite signal from space to the Earth, the signal will start off at full speed with there being no medium and then has it enters the Earths atmosphere and passes through the different densities of air of different altitudes, the light slows down in accordance with the density , so there would be a delay on the time of receiver compared to if there was no medium?

A time contraction?


 

Offline Thebox

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Re: Does air pressure slow light down?
« Reply #4 on: 05/07/2016 13:32:19 »
When air is dense (low pressure)  , does this slow light down compared to high pressure air (thin air)?
Are you sure you have your pressures the right way around? in my experience, air pressure is highest where it is densest (e.g. ground level), and lowest where it is thinnest (e.g. top of Everest). Light will be slower in the more dense air.
That's correct. Also cold air is denser and hot air less so which is why we get mirage effects close to a hot roadway.

Good one Box, pretty close.

Yes cold air is dense , because we know +hf=expansion and less dense and -hf=contraction and more dense.


So I had the high pressure and low pressure the right way around?

 

Online chiralSPO

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Re: Does air pressure slow light down?
« Reply #5 on: 05/07/2016 14:33:37 »
There are two issues here:

1) Air density: The ideal gas law PV = nRT (where: P = pressure, V = volume, n = number of gas molecules, R = a constant, T = temperature on an absolute scale) can be rearranged to give density as a function of temperature and pressure:

n/V = P/RT

So while temperature does change (mostly going lower as one goes higher in altitude, but also increasing in some parts of the atmosphere: http://www.windows2universe.org/kids_space/temp_profile.html), absolute temperature only varies between 300 K (surface temperature) and about 200 K (mesopause), but the pressure varies over several orders of magnitude, so the density falls by orders of magnitude as the altitude increases (Highest density near the ground.)

2) Influence of air density on speed of light:
Light gets slowed down as it interacts with matter. This is referred to as the index of refraction (vacuum = 1.0, air at Earth's surface = 1.0003, liquid water = 1.33, glass = 1.6, diamond, 2.6) The speed of light in that medium is equal to c/index of refraction, so light goes almost half as quickly through glass as it does through vacuum.

The difference between dense air and less dense air is very small: as light comes to Earth from space it slows from c to c/1.0003, which is fairly insignificant, but enough to matter for telescopes on the ground, and, as Colin points out, hot vs cold air is responsible for mirages (and the swirls/waves above a barbecue)

A simple kitchen experiment to look at this effect is to mix liquids that have different indices of refraction in a clear glass. I recommend filling a clear glass half way up with cool water from the faucet. To this one can slowly add one (or more) of the following:
vodka
grain alcohol
hot water
sugar water (pre-dissolve 1 tablespoon of sugar in 1/4 cup of water)
any other clear liquid that will dissolve in water

and watch what happens!
 

Offline Thebox

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Re: Does air pressure slow light down?
« Reply #6 on: 05/07/2016 14:46:15 »
There are two issues here:

1) Air density: The ideal gas law PV = nRT (where: P = pressure, V = volume, n = number of gas molecules, R = a constant, T = temperature on an absolute scale) can be rearranged to give density as a function of temperature and pressure:

n/V = P/RT

So while temperature does change (mostly going lower as one goes higher in altitude, but also increasing in some parts of the atmosphere: http://www.windows2universe.org/kids_space/temp_profile.html), absolute temperature only varies between 300 K (surface temperature) and about 200 K (mesopause), but the pressure varies over several orders of magnitude, so the density falls by orders of magnitude as the altitude increases (Highest density near the ground.)

2) Influence of air density on speed of light:
Light gets slowed down as it interacts with matter. This is referred to as the index of refraction (vacuum = 1.0, air at Earth's surface = 1.0003, liquid water = 1.33, glass = 1.6, diamond, 2.6) The speed of light in that medium is equal to c/index of refraction, so light goes almost half as quickly through glass as it does through vacuum.

The difference between dense air and less dense air is very small: as light comes to Earth from space it slows from c to c/1.0003, which is fairly insignificant, but enough to matter for telescopes on the ground, and, as Colin points out, hot vs cold air is responsible for mirages (and the swirls/waves above a barbecue)

A simple kitchen experiment to look at this effect is to mix liquids that have different indices of refraction in a clear glass. I recommend filling a clear glass half way up with cool water from the faucet. To this one can slowly add one (or more) of the following:
vodka
grain alcohol
hot water
sugar water (pre-dissolve 1 tablespoon of sugar in 1/4 cup of water)
any other clear liquid that will dissolve in water

and watch what happens!

Thank you for the informative post, so  light slows down the denser a medium, so could light be stopped by something that was so dense even light could not pass through it? 


 

Offline jeffreyH

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Re: Does air pressure slow light down?
« Reply #7 on: 05/07/2016 14:59:26 »
The density of the gravitational field at the event horizon of a black hole?
 

Offline Thebox

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Re: Does air pressure slow light down?
« Reply #8 on: 05/07/2016 15:12:44 »
The density of the gravitational field at the event horizon of a black hole?


Possible Jeff, a gravitational or magnetic field that was so dense, light was unable to surpass the ''shield'' in either direction of inwards or outwards .
 

Offline evan_au

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Re: Does air pressure slow light down?
« Reply #9 on: 05/07/2016 15:14:36 »
Quote from: TheBox
so  light slows down the denser a medium, so could light be stopped by something that was so dense even light could not pass through it?
In practice, when light strikes an object with a different index of refraction, you get partial reflection (eg when looking through glass).

When light strikes a material with a very different index of refraction, you get almost-total reflection, so the surface looks like a mirror.
 

Offline JohnDuffield

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Re: Does air pressure slow light down?
« Reply #10 on: 05/07/2016 15:21:36 »
Thank you for the informative post, so  light slows down the denser a medium
Yes, see the Shapiro delay:

"Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased by almost 2x10−4 sec when the radar pulses pass near the sun."

so could light be stopped by something that was so dense even light could not pass through it?
Like Jeffrey said, the at the event horizon of a black hole the coordinate speed of light is zero. People do then claim that "the local instantaneous proper speed of light is always c", but I'm afraid that's a popscience myth. Gravitational time dilation goes infinite at the event horizon. It would take an infinite time to measure the local speed of light at that location. It's wrong to claim that it's the same old 299,792,458 m/s at a place where a second lasts forever. 
 

Offline Thebox

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Re: Does air pressure slow light down?
« Reply #11 on: 05/07/2016 15:24:34 »


When light strikes a material with a very different index of refraction, you get almost-total reflection, so the surface looks like a mirror.

I would say that all ''matter'' mirrors light, some ''matter'' allows light to pass through more freely that is less wave compression compared to a body that has more dense structure that compresses the light creating a greater energy .

When I consider a reflection in a mirror, I consider the light is temporally stopped for an instant and there is a temporal distortion that creates the spectral content we see projected onto a mirror.

Am I correct in thinking that if we could slow light down to a stop or almost stop, we create plasma ?

And likewise is the imagine in a mirror a sort of temporal distortion and really low energy plasma?


Because surely we can turn water into steam and back steam back into water, so surely the plasma of the sun is turning into ''steam'' and recollected has ''water''?
 

Offline Thebox

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Re: Does air pressure slow light down?
« Reply #12 on: 05/07/2016 15:41:05 »
Thank you for the informative post, so  light slows down the denser a medium
Yes, see the Shapiro delay:

"Because, according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path, these time delays should thereby be increased by almost 2x10−4 sec when the radar pulses pass near the sun."

so could light be stopped by something that was so dense even light could not pass through it?
Like Jeffrey said, the at the event horizon of a black hole the coordinate speed of light is zero. People do then claim that "the local instantaneous proper speed of light is always c", but I'm afraid that's a popscience myth. Gravitational time dilation goes infinite at the event horizon. It would take an infinite time to measure the local speed of light at that location. It's wrong to claim that it's the same old 299,792,458 m/s at a place where a second lasts forever.

Just because a black hole is not seen to reflect photons by spectral content, this does not imply that light is not reflected, it just means there is no spectral content, so to say the speed of light is 0 at the event horizon maybe untrue, gravitational lensing shows light distortion to a comparison of ''translucent light''.


 

Online chiralSPO

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Re: Does air pressure slow light down?
« Reply #13 on: 05/07/2016 18:45:30 »
Thank you for the informative post, so  light slows down the denser a medium, so could light be stopped by something that was so dense even light could not pass through it?

It isn't possible to stop light entirely with a dense medium (a Bose-Einstein condensate), but people have slowed it down to the speed of a car:
http://physicsworld.com/cws/article/news/2009/dec/15/slowed-light-breaks-record
http://www.smithsonianmag.com/science-nature/putting-the-brakes-on-light-166960997/?no-ist
http://news.harvard.edu/gazette/1999/02.18/light.html

Note that slowing down light by interaction with matter like this is entirely different than gravitational effects on light--there is no change in clocks or time involved, as there would would gravity. Instead it is because of the magnetic and electric permitivity of the substances involved.
 

Offline jeffreyH

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Re: Does air pressure slow light down?
« Reply #14 on: 05/07/2016 23:36:40 »
Just to get one point straight. No one knows if the coordinate speed of light remains at zero for all future time. Hawking radiation is one consideration with black hole evaporation reducing the radius of the horizon over time. There may be others. This does not mean that proper time is zero at the event horizon.
 

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Re: Does air pressure slow light down?
« Reply #14 on: 05/07/2016 23:36:40 »

 

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