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I was always told that EM radiation is a far field effect. Does this

by cragar
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cragar
#1
Aug1-11, 04:55 AM
P: 2,466
I was always told that EM radiation is a far field effect. Does this mean that the light emitted from the accelerating electron is not right next to the electron but a little further out.
And also how do you calculate the frequency of the light coming off. When I looked through Griffiths it just seemed to talk about the total power radiated.
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chrisbaird
#2
Aug1-11, 10:26 AM
P: 617
Quote Quote by cragar View Post
I was always told that EM radiation is a far field effect. Does this mean that the light emitted from the accelerating electron is not right next to the electron but a little further out.
No, it means that the fields in close (near-field) to the accelerating electron are caused by the electron, but just have not taken the shape of regular traveling waves yet. You can think of near-field electromagnetic fields as the radiation fields plus other fields (pseudo-static fields, induction fields, etc.). So the radiation zone (far-field) is where we mean we are far enough away from the source that all other fields have died away and only the radiation field is left. Radiation fields (transverse traveling waves) are self-propagating, so they can just keep going without dieing out much. That is why we can see stars that are so far away.

The bottom line it that there isn't less going on in the near-field compared to the far-field, there is actually more going on. Also, causality still applies: em energy must pass through the near-field to get to the far-field. It can't just appear.

Quote Quote by cragar View Post
And also how do you calculate the frequency of the light coming off. When I looked through Griffiths it just seemed to talk about the total power radiated.
Figuring out the frequency is actually one of the easiest parts in calculating radiation. The radiated waves have the same frequency as the source. If I drive an antenna with a 10 MHz electric current, then the radio waves coming off will be at 10 MHz.
y33t
#3
Aug1-11, 11:33 AM
P: 107
Quote Quote by cragar View Post
I was always told that EM radiation is a far field effect. Does this mean that the light emitted from the accelerating electron is not right next to the electron but a little further out.
And also how do you calculate the frequency of the light coming off. When I looked through Griffiths it just seemed to talk about the total power radiated.
From a classical point of view electromagnetic radiation has two divisions for numerical evaluations ; far fields and near fields. The reason for this is that the behavior and components of the radiation are not same throughout it's path to target. Near fields are defined as the region enclosed by approximately a wavelength distance to the direction of propagation. Since both of them can induce each other ; components of electric and magnetic fields are not easily predictable in this region due to very small distances. Most of the today's computational electromagnetic tools have very low accuracy estimating near field behaviors of a modeled system.

Detecting frequency of an em radiation can be achieved by exposing it to an appropriate antenna and performing Fourier analysis of the induced electrical signal. Light is very small in wavelength thus it is not that easy to implement an appropriate 'antenna' to interact with visible spectrum. Even if optical antennas emerge and becomes practical, there is computing issue problem because it will require very fast processors to analyze PHz data stream and make real-time analysis. Considering a duty cycle ; frequency is not related to power in classical electromagnetics.

cragar
#4
Aug1-11, 06:23 PM
P: 2,466
I was always told that EM radiation is a far field effect. Does this

ok thanks for your answers, So far-field is where there is only the em radiation and no static fields. And when light is emitted from the electron is it always emitted next to the electron and then propagates out. Why can't it be emitted further out from the electron from its field energy.
chrisbaird
#5
Aug2-11, 10:09 AM
P: 617
Because of causality. If a cause happens at one point in space-time that creates an effect in another point in space-time, then information must have traveled between those two points.
cragar
#6
Aug2-11, 06:49 PM
P: 2,466
I see good point .
mikeph
#7
Aug3-11, 05:31 AM
P: 1,212
One questions regarding the frequency of emission... an electron oscillating at 10 MHz emits 10 MHz radiation - but it's not necessarily an oscillation that causes radiation, just an acceleration of charge. So what will the frequency of radiation be if we just accelerate it at, say 10 m s^-1? Some weird spectrum?
chrisbaird
#8
Aug3-11, 01:54 PM
P: 617
Quote Quote by MikeyW View Post
One questions regarding the frequency of emission... an electron oscillating at 10 MHz emits 10 MHz radiation - but it's not necessarily an oscillation that causes radiation, just an acceleration of charge. So what will the frequency of radiation be if we just accelerate it at, say 10 m s^-1? Some weird spectrum?
Yes, you get a smooth, continuous spectrum. Whether it is weird or not is up to personal interpretation. Radiation caused by a decelerating charged particle is called Bremsstrahlung (German for braking radiation) and by an accelerating point charge (non-relativistic) is called Larmor radiation.


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