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# 10.4: Electromagnetic Waves: Light

• • Contributed by Physics 7
• Physics at UC Davis

• 1. Harmonic Electromagnetic Waves
We knew that any electric charge created an electric field, and that any moving charge created a magnetic field. Other than that there is not much similarity: the forces produced by the electric field look very different than the forces produced by the magnetic field, and there are no "magnetic charges" (monopoles) for magnetic field lines to start or end on.  Then we started to discuss induction; we discovered that changing the magnetic flux through a loop caused an electric current to flow.
• 2. The Electromagnetic Spectrum
Taking charges (the source of electric fields) and oscillating them up and down should get the whole process started! The frequency with which charges oscillate up and down sets the frequency of the electromagnetic waves produced, like  similar to how the frequency of the wave on a string is set by a person at the end of the string, oscillating it. For electromagnetic waves, like all the other waves we have dealt with, the frequency is determined by the source.
• 3. Energy and Intensity of Light
While we have not emphasized it so far, electric and magnetic fields both contain energy. The total amount of energy depends on the values of the fields everywhere, so it is more convenient to define the energy density of the fields. This is the amount of energy per unit volume contained within the fields.  Unlike total energy, energy density can be defined easily for specific locations.  To obtain it for one position, we only need to know the value of the electric and magnetic fields at that po
• 4. Polarization and Polarizers
Polarization" may also mean the orientation of the plane of polarization.  Recall that the plane of polarization of the wave is the plane containing the direction of motion and the direction of electric field oscillation.  Light that is "polarized" in this sense is composed of electromagnetic waves that all oscillate in the same direction.  For example, vertically polarized light would have its electric field oscillating up-and-down.
• 5. Do We Need Fields?
Nothing we discussed so far seems to require fields; fields just made our job easier. One might be tempted to ask if fields actually exist, or if they're just convenient mathematical constructions.  To show why fields (or something like them) have to exist, consider moving a charge around for a short time. Energy is required to move the charge around, and the charge releases that energy to electromagnetic waves. Sometime later the wave hits some other charges, and starts them oscillating.
• 6. Summary