We can visualize the creation of EM waves as follows. If a charged particle accelerates (moves faster, slower or changes direction), it produces both an electric field (because the particle is charged) and a magnetic field (because the particle is moving). Furthermore because the motion of the particle is changing, the electric field is changing and the magnetic field is changing. The changing electric field creates a new magnetic field and the changing magnetic field produces a new electric field. The collapsing and regeneration of the electric and magnetic fields is what allows EM radiation to propagate. The animation below will help you visualize this process.
In this animation, an electromagnetic (EM) wave is generated by an accelerating electron. The z and x directions are on the plane of your screen and the y direction is coming out of the screen. As the electron moves up and down the associated electric field (represented by the black curve) also moves up and down. The electron motion also generates a magnetic field (magenta curve) coming out and going into the screen at right angles to the electron motion and the electric field. The magnetic field has the same sine wave shape as the electric field curve, but is distorted in the animation due to perspective. Because the particle's motion is changing (i.e. it is accelerating) the electric and magnetic fields are continually forming, collapsing and forming again in the opposite direction. The changing electric field generates another magnetic field to the right and the changing magnetic field generates another electric field to the right, thus the EM wave and associated force fields and energy to move in the direction of the black arrow at the speed of light. The small moving black and magenta arrows in the center of the animation represents the varying electric and magnetic force fields as the EM wave propagates past the green point.
Earlier we discussed how EM radiation also has the characteristics of a particle. We can think of it this way. When a charged particle, such as an electron accelerates, it releases a small packet of energy called a photon. Photons have no mass, they are pure energy. These photons moves along with the EM wave at the speed of light. The existence of photons has some important consequences, for example it means EM energy comes in tiny but finite packets that can't be divided. However for the purposes of this class, we will only need to consider the wavelike properties of EM radiation and the term "photon" will be used as just another term for an EM wave.
The EM wave represented in Figure 4 represents the wave field associated with one photon. In the real world, EM radiation consists of an enormous number of photons. If all the photon waves are oriented in the same way, for example, if all the electric fields are oriented along the vertical plane such as in the animated figure above, then the EM radiation is said to be polarized. EM radiation can be vertically polarized as above, or have some other polarization. If all the photon waves are oriented in different ways from each other, the radiation is unpolarized. However for each photon the electric and magnetic force fields will always be at right angles to each other, as shown above.
If all the peaks and valleys representing the electric and magnetic fields of different photons all line up with each other, the EM radiation is said to be in phase or coherent radiation. Most radio or radar transmissions are coherent. Visible and IR radiation is usually incoherent, an exception being lasers, which are coherent.
Charged particles create an electric force field. Moving charged particles create a magnetic force field. Accelerating charged particles produce changing electric and magnetic force fields which propagate as EM waves. EM radiation that has all the electric and magnetic field variations along the same plane
is polarized. Polarized radiation that is also in phase is coherent.
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