Question about electromagnetic waves?

[ChowPuppy]

Are the amplitudes of the electric and magnetic components of an electromagnetic wave proportional?
Or is the amplitude of the electric portion unrelated to that of the magnetic portion?

 

[Delta2]

E=cB is the relationship that holds between amplitudes, where E is the amplitude of the electric component and B that of the magnetic component. c is the speed of light.
If you want a bit more read the last 3 paragraphs of this wikipedia section http://en.wikipedia.org/wiki/Electromagnetic_wave#Derivation

 

[sophiecentaur]

E=cB is the relationship that holds between amplitudes, where E is the amplitude of the electric component and B that of the magnetic component. c is the speed of light.
If you want a bit more read the last 3 paragraphs of this wikipedia section http://en.wikipedia.org/wiki/Electromagnetic_wave#Derivation

Not quite accurate, actually. The E and H fields are related by

Z₀ = E/H
where Z₀ is the Impedance of free space (approx 377Ω)

Alternatively

E/B = Z₀/μ₀

I don't think your formula can be obtained by rearranging the above / substituting quantities.

http://en.wikipedia.org/wiki/Impedance_of_free_space" link shows it in more detail

 

[Delta2]

Not quite accurate, actually. The E and H fields are related by

Z₀ = E/H
where Z₀ is the Impedance of free space (approx 377Ω)

Alternatively

E/B = Z₀/μ₀

I don't think your formula can be obtained by rearranging the above / substituting quantities.

http://en.wikipedia.org/wiki/Impedance_of_free_space" link shows it in more detail

I think you are contradicting yourself here. In the link you give and in the derivation in the section "Relation to other constants" it uses the fact that E/B=c. Z₀/μ₀ equals the speed of light in vacuum! (as read in section "exact value" in the link you give).

 

[sophiecentaur]

Absolutely!
What a plonker! It's right in there, amongst all the other ratios.
Removes foot from mouth.

otoh, the concept of Free Space Impedance is, interestingly, also related to the ratio of fields. I'd have got away with it if I hadn't actually said you were wrong.

 

[Delta2]

This is news for me, i corrected a science advisor of Physics Forums!( Ok ok you weren't clearly wrong but rather self contradicting).
I think probably you didnt read at all the wiki link i gave, it eplains it all there and the only extra assumption made to derive the amplitude relation E=cB is that the direction of the E-field is constant that is the wave is linearly polarized.