Not understanding some parts of wave refraction
 

I am skimming thru the Propagation chapter of the ARRL handbook, and I
am having a difficult time understanding the shortening of wavelength
and the retainment of frequency. They have an equation showing that
wave velocity is: c = f*w (c = m/s, f = frequency, w = wavelength).
It also states that during refraction "the wavelength is
simultaneously shortened, but the wave frequency (number of crests
that pass a certain point in a given unit of time) remains constant."

I don't understand. If the wavelength is shortened, then shouldn't the
frequency increase instead of remaining constant?

Not understanding some parts of wave refraction
 

On 5 Apr 2007 07:36:49 -0700, "MRW" wrote:

c = f*w (c = m/s, f = frequency, w = wavelength)

This frequency is relevant ONLY for vacuum (or with a very, very
slight alteration) air.

Now, it may seem that all air is air, but no. There are slight
variations here too that on the global scale small shifts make large
changes. Those small shifts are accounted for by pressure, water
content (vapor), and temperature.

73's
Richard Clark, KB7QHC

Not understanding some parts of wave refraction
 

On Apr 5, 7:36 am, "MRW" wrote:
I am skimming thru the Propagation chapter of the ARRL handbook, and I
am having a difficult time understanding the shortening of wavelength
and the retainment of frequency. They have an equation showing that
wave velocity is: c = f*w (c = m/s, f = frequency, w = wavelength).
It also states that during refraction "the wavelength is
simultaneously shortened, but the wave frequency (number of crests
that pass a certain point in a given unit of time) remains constant."

I don't understand. If the wavelength is shortened, then shouldn't the
frequency increase instead of remaining constant?

Refraction occurs when an EM wave, having frequency f and wavelength w
enters a medium in which the speed of propagation (speed of light) is
different than vacuum. A medium with an index of refraction greater
than one produces a speed of light which is slower than in vacuum
(index of refraction is simply the ratio of vacuum speed to speed in
that medium). This changes the proportionality between frequency and
wavelength. Since w = c / f, the slower speed at a given frequency
will now have a correspondingly shorter wavelength. And, as f = c /
w, the slower speed at a given wavelength will now have a
correspondingly lower frequency.

I hope that makes sense.

73, Jim AC6XG

Not understanding some parts of wave refraction
 

MRW wrote:
I am skimming thru the Propagation chapter of the ARRL handbook, and I
am having a difficult time understanding the shortening of wavelength
and the retainment of frequency. They have an equation showing that
wave velocity is: c = f*w (c = m/s, f = frequency, w = wavelength).
It also states that during refraction "the wavelength is
simultaneously shortened, but the wave frequency (number of crests
that pass a certain point in a given unit of time) remains constant."

I don't understand. If the wavelength is shortened, then shouldn't the
frequency increase instead of remaining constant?

'c' decreases because of the fractional velocity
factor in a transmission line. The decrease in 'c'
compresses the wavelength but doesn't change the
frequency. 'c' is less in a transmission line than
it is in free space. The speed of light in RG-213,
for instance, is 2/3 of the speed of light in free
space.
--
73, Cecil, w5dxp.com

Not understanding some parts of wave refraction
 

MRW wrote:
I am skimming thru the Propagation chapter of the ARRL handbook, and I
am having a difficult time understanding the shortening of wavelength
and the retainment of frequency. They have an equation showing that
wave velocity is: c = f*w (c = m/s, f = frequency, w = wavelength).
It also states that during refraction "the wavelength is
simultaneously shortened, but the wave frequency (number of crests
that pass a certain point in a given unit of time) remains constant."

I don't understand. If the wavelength is shortened, then shouldn't the
frequency increase instead of remaining constant?

frequency stays the same, but since it's moving slower, c is smaller, so
lambda (wavelength) is shorter.

Same thing goes on in coaxial cable.. the wave propagates in a
dielectric with a propagation speed, say, 66% of the free space speed.
In such a case, a one wavelength long piece of coax for 30 MHz is 6.6
meters, not 10 meters (the free space wavelength)

The challenge, of course, would be in getting the opposite phenomenon to
occur (propagation faster than free space)...but that's a topic for a
different day.

jim

Not understanding some parts of wave refraction
 

On Thu, 05 Apr 2007 09:23:13 -0700, Jim Lux wrote:

MRW wrote:
I am skimming thru the Propagation chapter of the ARRL handbook, and I
am having a difficult time understanding the shortening of wavelength
and the retainment of frequency. They have an equation showing that
wave velocity is: c = f*w (c = m/s, f = frequency, w = wavelength).
It also states that during refraction "the wavelength is
simultaneously shortened, but the wave frequency (number of crests
that pass a certain point in a given unit of time) remains constant."

I don't understand. If the wavelength is shortened, then shouldn't the
frequency increase instead of remaining constant?

frequency stays the same, but since it's moving slower, c is smaller, so
lambda (wavelength) is shorter.

Same thing goes on in coaxial cable.. the wave propagates in a
dielectric with a propagation speed, say, 66% of the free space speed.
In such a case, a one wavelength long piece of coax for 30 MHz is 6.6
meters, not 10 meters (the free space wavelength)

The challenge, of course, would be in getting the opposite phenomenon to
occur (propagation faster than free space)...but that's a topic for a
different day.

jim

Speedy Gozales did it, but that's also a topic for a different day.

Walt, W2DU

Not understanding some parts of wave refraction
 

On Apr 5, 7:36 am, "MRW" wrote:
I am skimming thru the Propagation chapter of the ARRL handbook, and I
am having a difficult time understanding the shortening of wavelength
and the retainment of frequency. They have an equation showing that
wave velocity is: c = f*w (c = m/s, f = frequency, w = wavelength).
It also states that during refraction "the wavelength is
simultaneously shortened, but the wave frequency (number of crests
that pass a certain point in a given unit of time) remains constant."

I don't understand. If the wavelength is shortened, then shouldn't the
frequency increase instead of remaining constant?


Others have posted, correctly, that the propagation velocity is slower
in some mediums than in others. I think it's a mistake, though, to
say that c changes! c is supposed to be a constant, the speed of
electromagnetic wave propagation in a vacuum--in fact, I suppose, in a
vacuum with no gravitational fields in it. A description of fields in
an electromagnetic wave often used the permittivity, epsilon, and
permeability, mu, of the medium through which the wave is travelling.
If it's through a vacuum, the values of epsilon and mu have values
that are used often and have special notation--epsilon-sub-zero and mu-
sub-zero. For convenience here, call them eo and uo. Then note that
eo*uo = 1/c^2. As you might suspect, the propagation in a medium with
larger values of e and u than eo and uo is slower than c. In fact, it
should be velocity = sqrt(1/(e*u)).

Note that e has the units of capacitance/length -- commonly farads/
meter -- and u has the units of inductance/length -- commonly henries/
meter. But a farad is an ampere*second/volt, and a henry is a
volt*second/amp, so the units of sqrt(1/(e*u)) are sqrt(1/((A*sec/
V*meter)*(V*sec/A*meter))) = sqrt(meter^2/sec^2) = meters/sec. A unit
analysis is often useful to insure you haven't made a mistake in your
manipulation of equations.

So...in summary, c = f*w is actually not quite correct. It should be
wave_velocity = f*w. c should be reserved to mean only the speed of
light in a vacuum. If you're in a non-vacuum medium, and measure very
accurately, you'll measure the same frequency, but a shorter
wavelength: the wave doesn't travel as far to push a cycle past you,
as compared with in vacuum. It's going slower.

If the propagation medium is, for example, solid polyethylene (the
dielectric of most inexpensive coax cable), you'll find that w is
about 0.66 times as much as it is in a vacuum, and the propagation
velocity is similarly 0.66*c.

Cheers,
Tom

Not understanding some parts of wave refraction
 

On Apr 5, 1:44 pm, "K7ITM" wrote:
Others have posted, correctly, that the propagation velocity is slower
in some mediums than in others. I think it's a mistake, though, to
say that c changes! c is supposed to be a constant, the speed of
electromagnetic wave propagation in a vacuum--in fact, I suppose, in a
vacuum with no gravitational fields in it. A description of fields in
an electromagnetic wave often used the permittivity, epsilon, and
permeability, mu, of the medium through which the wave is travelling.
If it's through a vacuum, the values of epsilon and mu have values
that are used often and have special notation--epsilon-sub-zero and mu-
sub-zero. For convenience here, call them eo and uo. Then note that
eo*uo = 1/c^2. As you might suspect, the propagation in a medium with
larger values of e and u than eo and uo is slower than c. In fact, it
should be velocity = sqrt(1/(e*u)).

Note that e has the units of capacitance/length -- commonly farads/
meter -- and u has the units of inductance/length -- commonly henries/
meter. But a farad is an ampere*second/volt, and a henry is a
volt*second/amp, so the units of sqrt(1/(e*u)) are sqrt(1/((A*sec/
V*meter)*(V*sec/A*meter))) = sqrt(meter^2/sec^2) = meters/sec. A unit
analysis is often useful to insure you haven't made a mistake in your
manipulation of equations.

So...in summary, c = f*w is actually not quite correct. It should be
wave_velocity = f*w. c should be reserved to mean only the speed of
light in a vacuum. If you're in a non-vacuum medium, and measure very
accurately, you'll measure the same frequency, but a shorter
wavelength: the wave doesn't travel as far to push a cycle past you,
as compared with in vacuum. It's going slower.

If the propagation medium is, for example, solid polyethylene (the
dielectric of most inexpensive coax cable), you'll find that w is
about 0.66 times as much as it is in a vacuum, and the propagation
velocity is similarly 0.66*c.

Cheers,
Tom

Thank you everyone! I have a better understanding now. I guess part of
my confusion is that on the same chapter thay have a table on the
electromagnetic spectrum. In it, they list Radio Waves as having
frquencies between 10kHz to 300Ghz and wavelengths of 30,000km to 1mm
(I guess the 30,000 km is a typo in the book). Are these wavelength
values based in a vacuum then?

Not understanding some parts of wave refraction
 

K7ITM wrote:
On Apr 5, 7:36 am, "MRW" wrote:
I am skimming thru the Propagation chapter of the ARRL handbook, and I
am having a difficult time understanding the shortening of wavelength
and the retainment of frequency. They have an equation showing that
wave velocity is: c = f*w (c = m/s, f = frequency, w = wavelength).
It also states that during refraction "the wavelength is
simultaneously shortened, but the wave frequency (number of crests
that pass a certain point in a given unit of time) remains constant."

I don't understand. If the wavelength is shortened, then shouldn't the
frequency increase instead of remaining constant?


Others have posted, correctly, that the propagation velocity is slower
in some mediums than in others. I think it's a mistake, though, to
say that c changes! c is supposed to be a constant, the speed of
electromagnetic wave propagation in a vacuum--in fact, I suppose, in a
vacuum with no gravitational fields in it. A description of fields in
an electromagnetic wave often used the permittivity, epsilon, and
permeability, mu, of the medium through which the wave is travelling.
If it's through a vacuum, the values of epsilon and mu have values
that are used often and have special notation--epsilon-sub-zero and mu-
sub-zero. For convenience here, call them eo and uo. Then note that
eo*uo = 1/c^2. As you might suspect, the propagation in a medium with
larger values of e and u than eo and uo is slower than c. In fact, it
should be velocity = sqrt(1/(e*u)).

Note that e has the units of capacitance/length -- commonly farads/
meter -- and u has the units of inductance/length -- commonly henries/
meter. But a farad is an ampere*second/volt, and a henry is a
volt*second/amp, so the units of sqrt(1/(e*u)) are sqrt(1/((A*sec/
V*meter)*(V*sec/A*meter))) = sqrt(meter^2/sec^2) = meters/sec. A unit
analysis is often useful to insure you haven't made a mistake in your
manipulation of equations.

So...in summary, c = f*w is actually not quite correct. It should be
wave_velocity = f*w. c should be reserved to mean only the speed of
light in a vacuum. If you're in a non-vacuum medium, and measure very
accurately, you'll measure the same frequency, but a shorter
wavelength: the wave doesn't travel as far to push a cycle past you,
as compared with in vacuum. It's going slower.

If the propagation medium is, for example, solid polyethylene (the
dielectric of most inexpensive coax cable), you'll find that w is
about 0.66 times as much as it is in a vacuum, and the propagation
velocity is similarly 0.66*c.

Cheers,
Tom

Hi Tom -

That's certainly one way to look at it. (Though it is a little like
saying there is only one speed of sound.) Another way is to say that
c = 1/root(mu*epsilon) for any media. Light does after all, always
travel at the speed of light. ;-) Besides, it's more difficult to
explain Cherenkov radiation without the expression 'faster than the
speed of light in that medium'.

I thoroughly enjoyed the discussion you and Owen were (are) having
regarding amplifiers.
Thank you for that.

73, Jim AC6XG

Not understanding some parts of wave refraction
 

On Apr 5, 11:56 am, "Jim Kelley" wrote:
K7ITM wrote:
On Apr 5, 7:36 am, "MRW" wrote:
I am skimming thru the Propagation chapter of the ARRL handbook, and I
am having a difficult time understanding the shortening of wavelength
and the retainment of frequency. They have an equation showing that
wave velocity is: c = f*w (c = m/s, f = frequency, w = wavelength).
It also states that during refraction "the wavelength is
simultaneously shortened, but the wave frequency (number of crests
that pass a certain point in a given unit of time) remains constant."

I don't understand. If the wavelength is shortened, then shouldn't the
frequency increase instead of remaining constant?

Others have posted, correctly, that the propagation velocity is slower
in some mediums than in others. I think it's a mistake, though, to
say that c changes! c is supposed to be a constant, the speed of
electromagnetic wave propagation in a vacuum--in fact, I suppose, in a
vacuum with no gravitational fields in it. A description of fields in
an electromagnetic wave often used the permittivity, epsilon, and
permeability, mu, of the medium through which the wave is travelling.
If it's through a vacuum, the values of epsilon and mu have values
that are used often and have special notation--epsilon-sub-zero and mu-
sub-zero. For convenience here, call them eo and uo. Then note that
eo*uo = 1/c^2. As you might suspect, the propagation in a medium with
larger values of e and u than eo and uo is slower than c. In fact, it
should be velocity = sqrt(1/(e*u)).

Note that e has the units of capacitance/length -- commonly farads/
meter -- and u has the units of inductance/length -- commonly henries/
meter. But a farad is an ampere*second/volt, and a henry is a
volt*second/amp, so the units of sqrt(1/(e*u)) are sqrt(1/((A*sec/
V*meter)*(V*sec/A*meter))) = sqrt(meter^2/sec^2) = meters/sec. A unit
analysis is often useful to insure you haven't made a mistake in your
manipulation of equations.

So...in summary, c = f*w is actually not quite correct. It should be
wave_velocity = f*w. c should be reserved to mean only the speed of
light in a vacuum. If you're in a non-vacuum medium, and measure very
accurately, you'll measure the same frequency, but a shorter
wavelength: the wave doesn't travel as far to push a cycle past you,
as compared with in vacuum. It's going slower.

If the propagation medium is, for example, solid polyethylene (the
dielectric of most inexpensive coax cable), you'll find that w is
about 0.66 times as much as it is in a vacuum, and the propagation
velocity is similarly 0.66*c.

Cheers,
Tom

Hi Tom -

That's certainly one way to look at it. (Though it is a little like
saying there is only one speed of sound.) Another way is to say that
c = 1/root(mu*epsilon) for any media. Light does after all, always
travel at the speed of light. ;-) Besides, it's more difficult to
explain Cherenkov radiation without the expression 'faster than the
speed of light in that medium'.

I thoroughly enjoyed the discussion you and Owen were (are) having
regarding amplifiers.
Thank you for that.

73, Jim AC6XG


Hi Jim,

Some people may use only c-sub-zero for the speed of light in a
vacuum, but most commonly I see it simply as c, a fundamental physical
constant. To avoid confusion, I would HIGHLY recommend that either
you be very explicit that you're using co as the constant, and c as
the speed of light in whatever medium you're dealing with -- OR that
you're using c as the constant and whatever other notation for the
speed elsewhere.

NIST lists the constant both ways: c, c-sub-zero. SEVERAL other
places I just looked (reference books from my bookshelf; a web survey
including US, UK and European sites--mostly physics sites; several
university sites) only used c as the constant, except the NIST site
and one other, which both listed it as c or c-sub-zero with equal
weight.

It's clearly a matter only of notation, but I'll elect to stay with
the most commonly used notation, and from what I've seen just now,
most think c is a constant.

Cheers,
Tom

Not understanding some parts of wave refraction
 

On Apr 5, 11:33 am, "MRW" wrote:
On Apr 5, 1:44 pm, "K7ITM" wrote:



Others have posted, correctly, that the propagation velocity is slower
in some mediums than in others. I think it's a mistake, though, to
say that c changes! c is supposed to be a constant, the speed of
electromagnetic wave propagation in a vacuum--in fact, I suppose, in a
vacuum with no gravitational fields in it. A description of fields in
an electromagnetic wave often used the permittivity, epsilon, and
permeability, mu, of the medium through which the wave is travelling.
If it's through a vacuum, the values of epsilon and mu have values
that are used often and have special notation--epsilon-sub-zero and mu-
sub-zero. For convenience here, call them eo and uo. Then note that
eo*uo = 1/c^2. As you might suspect, the propagation in a medium with
larger values of e and u than eo and uo is slower than c. In fact, it
should be velocity = sqrt(1/(e*u)).

Note that e has the units of capacitance/length -- commonly farads/
meter -- and u has the units of inductance/length -- commonly henries/
meter. But a farad is an ampere*second/volt, and a henry is a
volt*second/amp, so the units of sqrt(1/(e*u)) are sqrt(1/((A*sec/
V*meter)*(V*sec/A*meter))) = sqrt(meter^2/sec^2) = meters/sec. A unit
analysis is often useful to insure you haven't made a mistake in your
manipulation of equations.

So...in summary, c = f*w is actually not quite correct. It should be
wave_velocity = f*w. c should be reserved to mean only the speed of
light in a vacuum. If you're in a non-vacuum medium, and measure very
accurately, you'll measure the same frequency, but a shorter
wavelength: the wave doesn't travel as far to push a cycle past you,
as compared with in vacuum. It's going slower.

If the propagation medium is, for example, solid polyethylene (the
dielectric of most inexpensive coax cable), you'll find that w is
about 0.66 times as much as it is in a vacuum, and the propagation
velocity is similarly 0.66*c.

Cheers,
Tom

Thank you everyone! I have a better understanding now. I guess part of
my confusion is that on the same chapter thay have a table on the
electromagnetic spectrum. In it, they list Radio Waves as having
frquencies between 10kHz to 300Ghz and wavelengths of 30,000km to 1mm
(I guess the 30,000 km is a typo in the book). Are these wavelength
values based in a vacuum then?


Clearly, the definition for the frequency range is somewhat
arbitrary. The boundary between infra-red and radio waves will
probably continue to be blurred as electronics advances further.

Radio waves down to much lower frequencies than 10kHz have been
used...the longer wavelengths penetrate water further, and are useful
for communicating with submarines. So don't be surprised if you come
across references to radio signals at 50Hz or so. Because
communications with radio waves is almost always based on propagation
through the vacuum of space, or through air which is only very
slightly slower, yes, the values for wavelength are based on c being a
constant, the speed of light in a vacuum.

Once you figure out one wavelength-frequency relationship, decade
(power-of-ten) values are easy:

1MHz = 300 meters (actually 299.792458, but almost universally taken
to be 300...)
10MHz = 30 meters
100MHz = 3 meters
etc...

Cheers,
Tom

Not understanding some parts of wave refraction
 

K7ITM wrote:


Hi Jim,

Some people may use only c-sub-zero for the speed of light in a
vacuum, but most commonly I see it simply as c, a fundamental physical
constant. To avoid confusion, I would HIGHLY recommend that either
you be very explicit that you're using co as the constant, and c as
the speed of light in whatever medium you're dealing with -- OR that
you're using c as the constant and whatever other notation for the
speed elsewhere.

NIST lists the constant both ways: c, c-sub-zero. SEVERAL other
places I just looked (reference books from my bookshelf; a web survey
including US, UK and European sites--mostly physics sites; several
university sites) only used c as the constant, except the NIST site
and one other, which both listed it as c or c-sub-zero with equal
weight.

It's clearly a matter only of notation, but I'll elect to stay with
the most commonly used notation, and from what I've seen just now,
most think c is a constant.

Cheers,
Tom

Hi Tom -

This is becoming circuitous. What you're saying is exactly what led
the original correspondent to be confused in the first place. Since
the relavant equation doesn't read c = f*w/n, the only way to explain
the phenomenon is by using a value of c that varies with medium. That
was the entire point.

73. Jim AC6XG

Not understanding some parts of wave refraction
 

K7ITM wrote:
It's clearly a matter only of notation, but I'll elect to stay with
the most commonly used notation, and from what I've seen just now,
most think c is a constant.

That's why I put it in quotes - to signify an unusual notation.
--
73, Cecil, w5dxp.com

Not understanding some parts of wave refraction
 

MRW wrote:

Thank you everyone! I have a better understanding now. I guess part of
my confusion is that on the same chapter thay have a table on the
electromagnetic spectrum. In it, they list Radio Waves as having
frquencies between 10kHz to 300Ghz and wavelengths of 30,000km to 1mm
(I guess the 30,000 km is a typo in the book). Are these wavelength
values based in a vacuum then?

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

Not understanding some parts of wave refraction
 

On Apr 5, 1:14 pm, Jim Kelley wrote:
K7ITM wrote:
Hi Jim,

Some people may use only c-sub-zero for the speed of light in a
vacuum, but most commonly I see it simply as c, a fundamental physical
constant. To avoid confusion, I would HIGHLY recommend that either
you be very explicit that you're using co as the constant, and c as
the speed of light in whatever medium you're dealing with -- OR that
you're using c as the constant and whatever other notation for the
speed elsewhere.

NIST lists the constant both ways: c, c-sub-zero. SEVERAL other
places I just looked (reference books from my bookshelf; a web survey
including US, UK and European sites--mostly physics sites; several
university sites) only used c as the constant, except the NIST site
and one other, which both listed it as c or c-sub-zero with equal
weight.

It's clearly a matter only of notation, but I'll elect to stay with
the most commonly used notation, and from what I've seen just now,
most think c is a constant.

Cheers,
Tom

Hi Tom -

This is becoming circuitous. What you're saying is exactly what led
the original correspondent to be confused in the first place. Since
the relavant equation doesn't read c = f*w/n, the only way to explain
the phenomenon is by using a value of c that varies with medium. That
was the entire point.

73. Jim AC6XG

Hi Jim,

OK, but I still say that, in that case, the equation (c=f*w) uses c in
a way that's inconsistent with common usage of c. I don't know if the
article quoted by the OP mentions that, or if somewhere it adds other
qualification, but if it's not out of context, then it would confuse
me, too, if I were trying to understand it for the first time. At the
very least, the article should say somewhere that c is the speed of
propagation in whatever medium we're dealing with, and if it did,
perhaps the OP wouldn't have been confused about it in the first
place. His posting makes it very clear to me that HE thought c was a
constant, as I would if the author didn't tell me otherwise.

Cheers,
Tom

Not understanding some parts of wave refraction
 

On 5 Apr 2007 18:15:30 -0700, "K7ITM" wrote:

HE thought c was a
constant, as I would if the author didn't tell me otherwise.

Hi Tom,

The speed of light is always constant - within its frame of reference.
It is only for those that inhabit a different frame that it "appears"
to be different. By Lorentzian laws, there is no time at the speed of
light and everything is simultaneous - source and load are
inseparable.

To illustrate at a slightly slower speed (from Feynman):
"A very interesting example of the slowing of time with motion
is furnished mu-mesons (muons), which are particles that
disintegrate spontaneously after an average lifetime of 2.2 µS.
They come to earth in cosmic rays.... It is clear that in its
short lifetime a muon cannot travel, even at the speed of light,
much more than 600 meters. But although the muons are created at
the top of the atmosphere, some 10 kilometers up, yet they are
actually found in a laboratory down here, in cosmic rays. How can
that be? The answer is that .... While from OUR own point of view
they live considerably longer ... time is increased ... by
1/SQRT(1-(u²/v²))."

73's
Richard Clark, KB7QHC

Not understanding some parts of wave refraction
 

Richard Clark wrote:
While from OUR own point of view
they live considerably longer ... time is increased ... by
1/SQRT(1-(u²/v²))."

I wonder what that implies about the alleged age
of the universe?
--
73, Cecil http://www.w5dxp.com

Not understanding some parts of wave refraction
 

On Apr 6, 12:08 am, Richard Clark wrote:
On 5 Apr 2007 18:15:30 -0700, "K7ITM" wrote:

HE thought c was a
constant, as I would if the author didn't tell me otherwise.

Hi Tom,

The speed of light is always constant - within its frame of reference.
It is only for those that inhabit a different frame that it "appears"
to be different. By Lorentzian laws, there is no time at the speed of
light and everything is simultaneous - source and load are
inseparable.

To illustrate at a slightly slower speed (from Feynman):
"A very interesting example of the slowing of time with motion
is furnished mu-mesons (muons), which are particles that
disintegrate spontaneously after an average lifetime of 2.2 µS.
They come to earth in cosmic rays.... It is clear that in its
short lifetime a muon cannot travel, even at the speed of light,
much more than 600 meters. But although the muons are created at
the top of the atmosphere, some 10 kilometers up, yet they are
actually found in a laboratory down here, in cosmic rays. How can
that be? The answer is that .... While from OUR own point of view
they live considerably longer ... time is increased ... by
1/SQRT(1-(u²/v²))."

73's
Richard Clark, KB7QHC

Seems to me you're way off point here, Richard. 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.
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. 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.
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...

OK, I'll shut up on this now.

Cheers,
Tom

Not understanding some parts of wave refraction
 

K7ITM wrote:
I also happen to notice
that the light from the distant star was created in a different
inertial frame of reference...

Not to mention being created in a different medium.
--
73, Cecil http://www.w5dxp.com

Not understanding some parts of wave refraction
 

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.

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

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).

Not understanding some parts of wave refraction
 

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

Not understanding some parts of wave refraction
 

K7ITM wrote:

"Let's see... e=m*c^2. Now what c is that?

I think I know which one, and as far as I know there hasn't been a
dispute about that on RRAA. The question posed on the newsgroup, and
the one which still seems to be a point of contention is which one
should go in *this* equation:

f = c / w

I maintain the answer is still - it depends on the medium.

I am sorry to have gotten so many pairs of panties wadded up about
this. Seemed a noncontroversial notion to me at the time.

73, Jim AC6XG

Not understanding some parts of wave refraction
 

Jim Kelley wrote in news:eve92l$mip$1
@news.service.uci.edu:



K7ITM wrote:

"Let's see... e=m*c^2. Now what c is that?

I think I know which one, and as far as I know there hasn't been a
dispute about that on RRAA. The question posed on the newsgroup, and
the one which still seems to be a point of contention is which one
should go in *this* equation:

f = c / w

I maintain the answer is still - it depends on the medium.

Jim, I am not a physicist... but I recall when introduced to e=m*c^2 at
high school, that "c" was defined as "the velocity of light in a vacuum".

If the "in a vacuum" qualification was unnecessary, if wasn't relevant, I
wonder why they complicated and restricted the definition?

I am with you (until someone presents a convincing argument otherwise).

This leaves all those books, software etc giving a value for "c" as not
necessarily in need of revision.

Owen

Not understanding some parts of wave refraction
 

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

That's the convention we used at Texas A&M
50 years ago.
--
73, Cecil http://www.w5dxp.com

Not understanding some parts of wave refraction
 

Being that I have zero qualifications as an expert on C I will tell
you how I see it.. pun intended

First, C is specified as the speed of photon travel in a vacuum, per
definition... Or as our attorney brethern say it, Per Se...

Second, C' is the measured speed of the propagation of the wave front
we call light through an atomic medium, air, water, glass, diamond,
etc....

The thing to keep in mind is that the two, C and C', are not identical
physical entities...
C is the unimpeded propagation of a photon - i.e. a single entity -
through a non atomic medium we call a vacuum - 'aether' if you will...
The photon that left Proxima Centauri ~4 years ago is the same photon
that slams into the front lens of the Hubble telescope (according to
that photon's wrist watch that ~4 years happened instantaneously, but
that's another rant)...
Now if you were in orbit at the eyepiece of the Hubble the photon that
enters your retina is not the same Proxima Centauri photon that
slammed into the Hubble's mirror... And that makes all the
difference...

C' happens when the photon smashes into the Hubble's glass and is
caught up in the electrical fields sorrounding the atoms making up the
glass, primarily the electrical field of the outer (valence) electrons
of the Protons... Like a house fly at full speed hitting a spiders web
the photon begins to tumble and veer sideways and rapidly decellerate
causing it to dump it's energy, i.e. radiate an electromagnetic field
of it's own... The electron's field absorbs the energy radiated off
the photon, causing in increase in the electrons energy level, usually
forcing it to jump to a higher 'orbit'...

Now at this instant the photon has disappeared (eaten by the spider)
and the electron has been pumped up (like Hulk Hogan)... After a short
interval the electron drops back to a lower energy level emitting a
new photon... This photon takes off like a scalded rabbit at C (or
very near C) but doesn't get far before another electron field sucks
it up again... And so it goes for the passage of the photon (actually
information) through the atomic structure of the transparent
material... The photon that pops out of the eyepiece and slams into
your retina is the lebenty sebenth generation descendent of the
original photon that hit your scope... The process of repeated
absorption and emission of a photon by multiple electrons is what
slows the light (wave front) when it travels through transparent
material... (and also what creates refraction, circles of confusion,
and a bunch of other phenomena)

Clear as mud, eh wot...

denny / k8do

Not understanding some parts of wave refraction
 

On Apr 5, 11:00 am, Richard Clark wrote:
On 5 Apr 2007 07:36:49 -0700, "MRW" wrote:

c = f*w (c = m/s, f = frequency, w = wavelength)

This frequency is relevant ONLY for vacuum (or with a very, very
slight alteration) air.

Now, it may seem that all air is air, but no. There are slight
variations here too that on the global scale small shifts make large
changes. Those small shifts are accounted for by pressure, water
content (vapor), and temperature.

73's
Richard Clark, KB7QHC

Same thing happens with light through water, the light slows down but
doesnt change in color(frequency).

Jimmie