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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? |
#2
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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 |
#3
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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 |
#4
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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 |
#5
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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 |
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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 |
#7
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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 |
#8
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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? |
#9
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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 |
#10
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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 |
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