# EM waves and fields

According to wikipedia, electromagnetic waves are "synchronized oscillations of electric and magnetic fields that propagate at the speed of light". I understand what it means in theory. But in practice we don't have a uniform EM field in nature, do we? EM field near a power station and in outter space might be quite different, does it influence the way EM waves (I'm thinking of light) propagates?

So in short: how is light influenced by EM fields on earth and what are the properties of EM field in absolute emptyness if that exists (in particular what is the intensity?).

• "how is light influenced by EM fields on earth?": light is EM waves – glS Jan 20 '15 at 14:17
• It sounds like you're really asking how electromagnetic waves interact with each other. Do you know anything about the Principle of Superposition or wave interference? – Sean Jan 20 '15 at 14:20
• @Sean in general I agree with you, but there exists such a thing as photon-photon interaction, at least theoretically. – Sofia Jan 20 '15 at 15:57

Inside semi-classical approach:

The EM waves from different sources do not interact in the absolutely empty space. They can just interfere in the sense that the local value of the field vector is dependent on the values of all the present partial vectors.

So, slowly oscillating (or constant) fields does not change the properties of the light wave in the empty space (pure vacuum).

However, the presence of matter changes the situation essentially. Almost all matter has more or less some essential interaction with EM field and therefore its properties could be changed by that. It means that the light wave passing through the matter will feel the influence of an external EM field in principle. The real cases of such interaction are dependent on strengths of those fields, type of matter and different conditions of interaction.

Intensity is energetic parameter of an any wave - it is portion of an energy that passed through the unit of a surface during the unit of a time in the process of a propagation of a wave.

• I probably have a misconception (but I am not a physicist). I try to reformulate: a wave is the propagation of a disturbance in a field (?). I know what a vector field is, mathematically. But in the physical world, is there one field to be disturbed? What are the properties of that field? If the disturbance has the frequency of light, then the physical effect will be emission of light, if it has a radio frequency then it will be emission of a radio wave... Or am I wrong in these statements? – Tom Jan 21 '15 at 7:23
• Yes, wave is a propagation of disturbance of a field. For example, you have charged particle producing permanent E-field around (Coulomb low). If you shake it, then E-field distribution around will be distorted and due to the finitness of an EM interation speed it will appear further at different moments. The field state in the case of harmonic distorsion will be like: E(R,t)=A*Cos[w(t-R/c)] Where A is Coulomb like amplitude, w-freq of the charge shake, c-speed of light, t and R are time and distance from charge. It is propagating wave. Wave could appear by ANY acelerations of a charges. – Rom38 Jan 21 '15 at 12:27

I think you are confused. First of all em waves are characterized by their wavelength. The thing which we call light is an em wave with a particular range of wavelength.(300-700 nm). Every thing around us absorb and give off energy in the form of em waves. Light is the only one which we can see. As for your question all em waves( be it light or not) travel at the same speed irrespective of their source. Em waves are like individual quantities. They dont belong to earth nor to space. Their interaction is not in the way you are thinking. Whatever properties that you observe of light/em waves on earth will be same everywhere.