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We know that light is an electromagnetic wave and it does interact with charges.

It contains magnetic field and electric field oscillating perpendicularly but when we apply an electric or magnetic field in any direction to the wave the applied electric field or magnetic field vector doesn't alter the magnetic or electric field in the electro magnetic wave (according to vector addition rule)....why?

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  • $\begingroup$ Hi Kench. Welcome to Physics SE. Consider reformulating your question in a clearer way $\endgroup$
    – Andrea
    Commented Jan 28, 2016 at 14:06
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    $\begingroup$ Are you asking why electromagnetic field is not affected by other electromagnetic field? That is because electromagnetic field is governed by a linear equation, so it does not contain self-interaction. $\endgroup$
    – mpv
    Commented Jan 28, 2016 at 14:29

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An applied electric or magnetic field doesn't alter the field of an electromagnetic field because, as you said, the superposition principle holds. This principle is a principle of linearity, and comes from the linearity of electromagnetic equations : there is no interaction between photons at low energies.

You can see it from a field theory point of view, as there is no bare interaction vertex between photons in QED.

On the other hand, in other theories such as QCD, gauge bosons (the gluons) carry a colour charge and can interact.

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    $\begingroup$ Here is a photon-photon interaction. en.wikipedia.org/wiki/Schwinger_limit#/media/… $\endgroup$
    – Mikael Kuisma
    Commented Jan 28, 2016 at 14:34
  • $\begingroup$ As you use the term "photons" you should qualify: there is very small probability of photon photon interactions. This goes up with energy, they are discussions of a gamma gamma collider slac.stanford.edu/pubs/beamline/26/1/26-1-kim.pdf $\endgroup$
    – anna v
    Commented Jan 28, 2016 at 14:45
  • $\begingroup$ You're right, my mistake. I should have said at low energy (usual condition for linearity). $\endgroup$
    – Dimitri
    Commented Jan 28, 2016 at 14:49
  • $\begingroup$ This answer is correct, but it holds only for fields in the vacuum, and as @annav points out, only low energy. So yes, it is correct, but not really. :-) $\endgroup$
    – garyp
    Commented Dec 16, 2020 at 22:22
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We know that light is an electromagnetic wave and it does interact with charges.

It contains magnetic field and electric field oscillating perpendicularly but when we apply an electric or magnetic field in any direction to the wave the applied electric field or magnetic field vector doesn't alter the magnetic or electric field in the electro magnetic wave (according to vector addition rule)....why?

Static electric and magnetic fields do affect electromagnetic waves, and one trusts that the mathematics works, vector additions and all. The conditions are studied in plasmas, for example this model.

Generally, when light scatters or diffracts through a crystal the electric fields of the wave are perturbed and change direction, become polarized or whatever the conditions are. See as an example Thomson scattering for elastic scattering of light .

At the quantum mechanical level there exists a scattering of photons with charged particles, and be assured that at the limit of the emergent classical beam the calculations will agree.

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In fiber optics communications, we know that when we add a new wavelength very close to other wavelengths already sending information within the same fiber (i.e. within 1 nm of each other), there is a large disruption downstream. This causes large amounts of disruptions in communications. But if we alter the electric field in adjacent wavelengths to be orthogonal to each other, the downstream disruption is significantly reduced.

This implies that the electric fields do interact.

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