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Author Topic: Maxwell Electromagnetic theory of light  (Read 2666 times)

Offline copernicus1234

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Maxwell Electromagnetic theory of light
« on: 06/05/2014 21:55:29 »
I was looking at a physics book and found that Maxwell's electromagnetic theory of light is based on Ampere and Faraday induction experiments yet induction is not optical.  Can someone, that is smart, explain this to me?  How can electromagnetic induction be used to describe an electromagnetic field structure when the described induction experiments are not optical.  Does the induction experiments have to be optical to represent a structure with equations?


 

Offline JP

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Re: Maxwell Electromagnetic theory of light
« Reply #1 on: 06/05/2014 22:42:27 »
Actually, you've struck on the most amazing part of Maxwell's equations.  It was known for a long time that charges created electric fields and moving charges (currents) created magnetic fields.  What Faraday showed is that a magnetic field that changes in time creates an electric field with his induction experiments.

Some time later, Maxwell came along and showed by a thought experiment that the reverse also should be true: a time varying electric field creates a magnetic field. 

All of these could be confirmed experimentally in the lab with magnets, flowing currents and charged objects. 

Maxwell then realized that taken together, these equations implied that electric and magnetic fields can exist independently of charges and travel as waves.  (Essentially in a wave, the electric and magnetic fields both vary in time so that they sustain each other).  The truly amazing part was that certain constants that had been found by measuring the strengths of electric and magnetic fields in the lab combined in his equations to predict that this electromagnetic wave moved at exactly the speed of light.  (The speed of light had been measured in other experiments).  This led him to conclude that light must be a form of electromagnetic wave, which is something we know today to be true.
 

Offline copernicus1234

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Re: Maxwell Electromagnetic theory of light
« Reply #2 on: 07/05/2014 00:04:54 »
The velocity of the radio induction effect cannot be used to justify Maxwell's Electromagnetic theory of light since induction is not optical.  In addition, the derivations of Maxwell's EM wave equations of light using the expansion and gradient methods are patently incorrect.  Unless you believe in scalar EM waves.
 

Offline JP

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Re: Maxwell Electromagnetic theory of light
« Reply #3 on: 07/05/2014 12:26:57 »
The velocity of the radio induction effect cannot be used to justify Maxwell's Electromagnetic theory of light since induction is not optical.  In addition, the derivations of Maxwell's EM wave equations of light using the expansion and gradient methods are patently incorrect.  Unless you believe in scalar EM waves.

You have to use all of Maxwell's equations to justify it (at least the four equations called Maxwell's equations in modern notation), not just induction.

What you can do is to justify that whatever this EM field it, it moves at a speed that is coincidentally exactly the same as the speed of light (at least to within measurement errors) both within materials and in vacuum.

Separate experiments since have shown that light also follows Maxwell's equations.

I'm not sure what you mean by "expansion and gradient methods," but typical derivations of the wave equation from Maxwell's equations rely heavily on the vector nature of the fields.
 

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Re: Maxwell Electromagnetic theory of light
« Reply #3 on: 07/05/2014 12:26:57 »

 

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