Discussion in 'Physics' started by lendo1, Apr 25, 2010.

1. ### lendo1 Thread Starter Active Member

Apr 24, 2010
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Hi I'm Leo, I'm new to the forum .

Anyways, I have been perplexed by an MIT lecture. Is anyone able to explain why the radio waves pass through the metal bars in the direction perpendicular to their oscillation but not parallel?

41:00 - 48:00

The specific part with the bars is described and tested at 45:00

Thanks

Apr 24, 2010
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3. ### Wendy Moderator

Mar 24, 2008
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It goes with polarization in general, how the magnetic waves are aligned. The electric and magnetic waves are 90° apart, I could have easily talked about how the electric wave is aligned. Together they make a photon.

A polarized lens, the kind found in sun glasses, is exactly the same case. The difference between light and radio is only that of frequency, light can also be polarized.

An interesting experiment you can try is take a laser pointer and shine it though polarized glasses. Rotate the laser pointer with respect to the lens. Since solid state lasers in specific (and I think lasers in general) are polarized you will see the beam passed through or blocked according to angle.

4. ### lendo1 Thread Starter Active Member

Apr 24, 2010
34
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Thanks Bill, maybe I over-analyzed the problem. I do understand polarization of EM waves in general, but I still do not understand the reason for the metal bars perpendicular to the oscillation of the charges to admit the charge. Is this perhaps because when the bars are parallel to the oscillation they absorb the radiation like an antenna?

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I guess if you think of the wave Electric and Magnetic components oscillating either in the direction of the bars or orthogonal to them you have the two options.

If the E component oscillates parallel to the line of the bars and the M component oscillates perpendicular to the line the bars then this is the most likely case for energy absorption in the bars.

In the alternate case where E oscillates perpendicular to the line of the bars and M oscillates parallel to the line of the bars you have the least likely case for energy absorption.

6. ### BillO Distinguished Member

Nov 24, 2008
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The bars perpendicular to the polarization do not interact at all with the wave. They are invisible to it and do not have any current induced in them because of it. Like they were not there. The bars parallel to the polarization fully interact with the wave. The energy carried by the wave induces a current in them and is absorbed. As long as the parallel bars are less than the wavelength apart they will be opaque to the wave.

7. ### davebee Well-Known Member

Oct 22, 2008
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I don't disagree that the case of parallel bars will be opaque to RF, but have two questions -

Won't much of the RF be re-radiated as opposed to being absorbed?

And if RF energy is re-radiated, then what prevents it from being re-radiated in the forward direction?

8. ### nsaspook AAC Fanatic!

Aug 27, 2009
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It would mainly just be adsorbed into heat or just reflected if smaller than the wavelength of the bars.

If the bars were tuned to the frequency the radiation pattern could be anything, forward, backwards depending on the "receiving" bar antenna.

9. ### lendo1 Thread Starter Active Member

Apr 24, 2010
34
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Alright I see now, I was thinking about the oscillating electric field inducing a current instead of the magnetic field. Great responses !

10. ### ELECTRONERD Senior Member

May 26, 2009
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One thing that's always remained tentative to me, is the fact that EM waves are always perpindicular; even in a vacuum. What causes that to happen?

Austin

11. ### Wendy Moderator

Mar 24, 2008
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I suspect it is a fundimental property, a photon must be this way to be a photon.

Aug 27, 2009
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