Electromagnetic waves

What are electromagnetic waves? Light, microwaves, x-rays, and radio transmissions are all kinds of electromagnetic waves. The different names refer to different wavelengths as shown Transmission spectra of the earth's atmosphere (Fig. 1.3)

Figure 1.3: Transmission spectra of the atmosphere, after [Kronenberg85]

In this case the wave is not created by a hand moving a rope up and down like in the mechanical case described above. Instead, the wave is created by moving charged particles in an antenna creating a so called electric force field. In physics, a force field is a way to picture the effects that electric charges have on one another. We can say a charge creates a force "field" in the empty space around it. a second charge with the same sign (++ -) put down at any place in this force field is pushed away, a charge with opposite sign (+-) set down at the same place is pulled towards. One can visualize "lines" of force by looking at the forces created by the field in many different places; imagine connecting the lines from all the electrons you have placed. The lines in such a pattern are known as "lines of force." One may imagine those lines as the connection between the elements like the rope in the mechanical case. Moving one charge causes the field lines attached to it to wiggle, and after a time the other charge starts to wiggle. Therefore, the wave here consists of a wiggling line of electric force which you can think of as being "attached" to the vibrating charge. In case of an antenna a great number of electrons are moving. As a result loops of electric lines of force are formed which move away from the charges along rays (Fig. 1.4).

Figure 1.4: Transmission of an electromagnetic wave by a dipole

In case of the electromagnetic wave the two energy forms which are exchanged are not potential and kinetic energy, but electric and magnetic fields. It can be shown (see Appendix Maxwell equations) that these fields are coupled and form together the electromagnetic wave. Since the electrical and magnetic field of an electromagnetic wave are coupled (see Appendix Maxwell equations) the wave can be described entirely be the electromagnetic field vector $\vec{E}$.