Emission of EM Wave: Why Does it Happen?

[justwild]

In wikipedia's description of transmitter, if an alternating current is given to an antenna, then the antenna radiates off em wave.
But why this happens so?

 

[Simon Bridge]

Nobody knows - "why" questions are not what science does.

However - the EM waves comes of an accelerating electron because electricity and magnetism are part of the same force. The force is described by Maxwell's equations.
The exact description depends on which model you want to use - so some sort of context would hep here.

 

[jtbell]

The simplistic description is this:

1. An electric current produces a magnetic field.

2. An oscillating (alternating) electric current produces an oscillating magnetic field.

3. An oscillating magnetic field "produces" an oscillating electric field, which in turn "produces" more oscillating magnetic fields, which in turn "produce" more oscillating electric fields, etc. via the time-derivative terms in Maxwell's equations. These oscillations propagate at speed c.

A more accurate version of step 3 is that oscillating magnetic fields are inevitably associated with oscillating electric fields, both propagating at speed c, again via the time-derivative terms in Maxwell's equations. This avoids the implication of a circular cause and effect chain.

 

[Neandethal00]

The simplistic description is this:

1. An electric current produces a magnetic field.

2. An oscillating (alternating) electric current produces an oscillating magnetic field.

3. An oscillating magnetic field "produces" an oscillating electric field, which in turn "produces" more oscillating magnetic fields, which in turn "produce" more oscillating electric fields, etc. via the time-derivative terms in Maxwell's equations. These oscillations propagate at speed c.

A more accurate version of step 3 is that oscillating magnetic fields are inevitably associated with oscillating electric fields, both propagating at speed c, again via the time-derivative terms in Maxwell's equations. This avoids the implication of a circular cause and effect chain.

Ok, oscillating electric current can produce electromagnetic waves from an antenna. Frequency of generated electromagnetic wave is the same as frequency of oscillations of current in the antenna. I think I'm correct on my last sentence.

Then I think it is also possible to observe visible light from an antenna by controlling frequency of oscillations of its current.

Just a thought.

 

[Drakkith]

Ok, oscillating electric current can produce electromagnetic waves from an antenna. Frequency of generated electromagnetic wave is the same as frequency of oscillations of current in the antenna. I think I'm correct on my last sentence.

Then I think it is also possible to observe visible light from an antenna by controlling frequency of oscillations of its current.

Just a thought.

Not from a standard antenna. We simply can't oscillate the electrons back and forth at a high enough frequency, it's just way too high! Perhaps future technology will have the ability to do so, and I think I might have read something about nano-scale antennas that can do this, but I really don't know.

 

[Simon Bridge]

Yah. In principle one could oscillate charges to get optical EM fields ... particularly in the classical model considered here. However, the material properties of the antenna are also important.

We usually exploit some other properties instead ... like heating the metal until it glows.
There is a point where the classical "light wave" model stops being useful.