MRW wrote:
On Apr 5, 4:40 pm, Roy Lewallen wrote:
Yes. And it's very, very nearly the same for air.

The 30,000 km would be a typo -- the wavelength in a vacuum at 10 kHz
would be 30 km.

Roy Lewallen, W7EL

Thanks again everyone! It makes sense to me to just treat c, in this
case, as a relative speed dependent on the medium.

As others have pointed out, it's risky to treat c as a variable or
medium-dependent speed. That letter is nearly always used to designate
the speed of light (or any EM plane wave) in a vacuum. Using that
nearly-universal definition, the speed of an EM wave in any other medium
is VF * c where VF is the "velocity factor". It's important to realize
that while there's a single value for the speed of all EM waves in a
vacuum (c), this isn't true in many other media. In many media, the
speed of the wave depends on its frequency, a phenomenon called
"dispersion". So in many media there's no universal EM velocity
equivalent to c, but rather a frequency-dependent velocity factor.

In environments where the field is confined such as a waveguide, the
velocity can also depend on the mode, that is the orientation of the
fields. So there's not even a single value for each frequency. And this
can be true even if the waveguide is filled with a vacuum.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Using that
nearly-universal definition, the speed of an EM wave in any other medium
is VF * c where VF is the "velocity factor".

If I remember correctly, at Texas A&M we used an
equation like:

c' = VF(c)

Writing c-prime like that told us that it wasn't
the speed of light in free space.
--
73, Cecil http://www.w5dxp.com

Roy Lewallen wrote:

As others have pointed out, it's risky to treat c as a variable or
medium-dependent speed.

Roy -

The convenient thing about using medium dependent c in that equation
is that we can use things such as index of refraction or velocity
factor to convert from vacuum 'c' to 'c' in another medium. The fact
that it makes the results of the calculation more accurate tends to
mitigate any risk that might be encumbered when using it.

To require that only vacuum c be used in the equation to me seems
overly authoritarian. I wonder how you feel about the speed of sound? :-)

73, Jim AC6XG


That letter is nearly always used to designate
the speed of light (or any EM plane wave) in a vacuum. Using that
nearly-universal definition, the speed of an EM wave in any other medium
is VF * c where VF is the "velocity factor". It's important to realize
that while there's a single value for the speed of all EM waves in a
vacuum (c), this isn't true in many other media. In many media, the
speed of the wave depends on its frequency, a phenomenon called
"dispersion". So in many media there's no universal EM velocity
equivalent to c, but rather a frequency-dependent velocity factor.

In environments where the field is confined such as a waveguide, the
velocity can also depend on the mode, that is the orientation of the
fields. So there's not even a single value for each frequency. And this
can be true even if the waveguide is filled with a vacuum.

Roy Lewallen, W7EL

On 6 Apr 2007 09:04:59 -0700, "K7ITM" wrote:
Seems to me you're way off point here, Richard.

Hi Tom,

Hardly an unfamiliar comment.

I'm in my lab, my
inertial frame of reference. I send some EM waves through my vacuum
chamber and I measure their speed as 2.997...*10^8 meters/second. The
same waves continue on through the glass of the bell jar keeping air
out of my vacuum, and I happen to notice that their speed through that
glass is1.684*10^8 meters/second. I notice that light from my
hydrogen light source contains certain well-defined spectral lines,
but each of those passes through my vacuum at the same speed.

Same as what?

However, I notice that those lines, in a short pulse of light, come
out of the glass separated in time slightly, implying that they took
different times to get through the glass, and were therefore not even
travelling through the glass at the same velocity; I notice no such
separation for the light passing through the vacuum.

Same as what? Different wavelengths respond to different inertial
frames of reference, a prism demonstrates this quite dramatically.
This happens quite commonly for wideband transmissions through fiber
optics. The solution has been to send them as Soliton waves. Another
solution is to employ micro channels.

Further, I
notice that light from a distant star has apparently the same set of
spectral lines, but they are shifted to slightly longer wavelengths.

A typical frame of reference example.

However, they take the same time to pass through the vacuum as my
locally-generated hydrogen light.
All my measurements are in the same
frame of reference, and IN VACUUM the speed of em radiation appears
from all my measurements to be the same, no matter its wavelength,
even for very long wavelengths, but in other media, still the same
inertial reference frame, it's different. I also happen to notice
that the light from the distant star was created in a different
inertial frame of reference...

There are a lot of "same"s here and some are being shown to be
different, and others same. I'm wondering what the point is that I'm
way off from.

73's
Richard Clark, KB7QHC

Jim Kelley wrote:
Roy Lewallen wrote:

As others have pointed out, it's risky to treat c as a variable or
medium-dependent speed.

Roy -

The convenient thing about using medium dependent c in that equation is
that we can use things such as index of refraction or velocity factor to
convert from vacuum 'c' to 'c' in another medium. The fact that it
makes the results of the calculation more accurate tends to mitigate any
risk that might be encumbered when using it.

To require that only vacuum c be used in the equation to me seems overly
authoritarian. I wonder how you feel about the speed of sound? :-)

73, Jim AC6XG

What, the speed of sound in a vacuum? I'm afraid you'll have to ask
Cecil or Art about that -- I'm not qualified to comment.

I'm not trying to be authoritarian about the use of "c", just reporting
what I find in my textbooks. Grabbing just one for example, Kraus'
_Electromagnetics_, on p. 352 I find that he uses v as the phase
velocity, and says, "For free space (vacuum) the velocity is a
well-known constant (usually designated by c and usually called the
velocity of light)." and shows an equation for c. Then he gives an
equation for the "relative phase velocity" p, as v/c.

In the back of R.K. Moore's _Traveling-wave Engineering_, c is listed as
"Velocity of light in vacuum". He uses v-sub-p for phase velocity.

A number of authors avoid using c altogether, but those who do seem to
universally use it to mean the speed of light in a vacuum. What texts do
you have where it's used to mean the phase velocity in a medium other
than air?

Of course, you can always go ahead and interpret c any way you want,
even if it isn't what the author intended. Then you can progress from
there to any number of bizarre conclusions. They'd fit right in with the
ones being "debated" over and over on this newsgroup.

Roy Lewallen, W7EL

On Fri, 06 Apr 2007 16:29:07 -0700, Jim Kelley wrote:

Roy Lewallen wrote:

As others have pointed out, it's risky to treat c as a variable or
medium-dependent speed.

Roy -

The convenient thing about using medium dependent c in that equation
is that we can use things such as index of refraction or velocity
factor to convert from vacuum 'c' to 'c' in another medium. The fact
that it makes the results of the calculation more accurate tends to
mitigate any risk that might be encumbered when using it.

To require that only vacuum c be used in the equation to me seems
overly authoritarian. I wonder how you feel about the speed of sound? :-)

73, Jim AC6XG

The speed of sound in a vacuum is measured using the sound of one hand clapping.

Walt

Roy Lewallen wrote:

Jim Kelley wrote:

Roy Lewallen wrote:

As others have pointed out, it's risky to treat c as a variable or
medium-dependent speed.


Roy -

The convenient thing about using medium dependent c in that equation
is that we can use things such as index of refraction or velocity
factor to convert from vacuum 'c' to 'c' in another medium. The fact
that it makes the results of the calculation more accurate tends to
mitigate any risk that might be encumbered when using it.

To require that only vacuum c be used in the equation to me seems
overly authoritarian. I wonder how you feel about the speed of sound?
:-)

73, Jim AC6XG


What, the speed of sound in a vacuum? I'm afraid you'll have to ask
Cecil or Art about that -- I'm not qualified to comment.



I'm not trying to be authoritarian about the use of "c", just reporting
what I find in my textbooks. Grabbing just one for example, Kraus'
_Electromagnetics_, on p. 352 I find that he uses v as the phase
velocity, and says, "For free space (vacuum) the velocity is a
well-known constant (usually designated by c and usually called the
velocity of light)." and shows an equation for c. Then he gives an
equation for the "relative phase velocity" p, as v/c.



In the back of R.K. Moore's _Traveling-wave Engineering_, c is listed as
"Velocity of light in vacuum". He uses v-sub-p for phase velocity.

A number of authors avoid using c altogether, but those who do seem to
universally use it to mean the speed of light in a vacuum. What texts do
you have where it's used to mean the phase velocity in a medium other
than air?

Of course, you can always go ahead and interpret c any way you want,
even if it isn't what the author intended. Then you can progress from
there to any number of bizarre conclusions. They'd fit right in with the
ones being "debated" over and over on this newsgroup.

Roy Lewallen, W7EL

I think we both understand that light travels at a velocity which is
dependent on the medium through which it is travelling. You seem to
want to continue to argue about that, and to tell you the truth I
can't see much difference between that, and the kind of debate going
on over and over in this newsgroup.

73, Jim AC6XG

Jim Kelley wrote:

I think we both understand that light travels at a velocity which is
dependent on the medium through which it is travelling. You seem to
want to continue to argue about that, and to tell you the truth I can't
see much difference between that, and the kind of debate going on over
and over in this newsgroup.

This isn't the first time I've failed to communicate, and I'm sure it
won't be the last.

The sole point I was trying to make is that the letter c is just about
universally used, as far as I can tell, to mean the velocity of light in
a vacuum. That symbol is not generally used to mean the speed of light
in any other medium.

Roy Lewallen

Cecil Moore wrote:
Roy Lewallen wrote:
Using that nearly-universal definition, the speed of an EM wave in any
other medium is VF * c where VF is the "velocity factor".

If I remember correctly, at Texas A&M we used an
equation like:

c' = VF(c)

Writing c-prime like that told us that it wasn't
the speed of light in free space.
my recollection from optics is

c/c' = n, index of refraction for the c' medium, (c always used as the
constant in a vacuum).

On Apr 9, 12:20 pm, Roy Lewallen wrote:
Jim Kelley wrote:

I think we both understand that light travels at a velocity which is
dependent on the medium through which it is travelling. You seem to
want to continue to argue about that, and to tell you the truth I can't
see much difference between that, and the kind of debate going on over
and over in this newsgroup.

This isn't the first time I've failed to communicate, and I'm sure it
won't be the last.

The sole point I was trying to make is that the letter c is just about
universally used, as far as I can tell, to mean the velocity of light in
a vacuum. That symbol is not generally used to mean the speed of light
in any other medium.

Roy Lewallen


I'm with Roy on this. v is a wonderful symbol for generalized
velocity; c is used by far too many--as Roy says, almost universally--
as the velocity of light in vacuum. Speaking of failures to
communicate, I think using a symbol that's so well accepted to mean
one thing when you mean something else is a wonderful way to
precipitate failures of communication. Clear communication is aided
by not changing the meaning of well-accepted symbols.

"Let's see... e=m*c^2. Now what c is that? Is it 3e8m/s, or is it
only 3.4e7m/s because I'm in water? Will the nuclear blast be only
1/78 as energetic because it's conducted in water? Oh, but wait, at
higher frequencies c is greater than at low frequencies, in water.
Oh, I'm getting soooo confused...." No, c=3e8m/s, nominally.

"Let's see... epsilon-zero = 1/(mu-zero * c^2). Now what c is
that? ...."

Examples of equations where c is taken for granted as the freespace
speed of light abound. I think publishing an equation where c is the
speed of light, but not necessarily the freespace speed, shows poor
technical editing.



Cheers,
Tom