1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How light starts

  1. Mar 17, 2010 #1
    I've been reading my physics book about Maxwell's equations. It makes sense to me that an electric and magnetic wave could be self supportive and vary with time. What doesn't make sense to me is why it should travel through space, in other words vary in space. The derivation of the wave equation from Maxwell's equations is very suggestive if not proof. I just want to know how you could know where the wave begins and what starts it. And if non varying electric and magnetic fields start it and are present far away instantaneously, why don't the electromagnetic waves start far away as well. The book I'm reading only explains it by saying that the magnetic and electric fields start the waves nearby and don't give a reason why they should think that. Do all changes in values of electric or magnetic fields have to propagate out at the speed of light or do places far away change instantaneously. Maybe I should differentiate between electric fields caused by changing magnetic fields from electric fields caused by charges but electrodynamic books don't seem to differentiate.
  2. jcsd
  3. Mar 17, 2010 #2


    Staff: Mentor

    The easiest way to think of this is to consider a standard dipole antenna. In a dipole antenna you have a current sloshing back and forth inside the antenna, but the current can't leave the ends of the antenna, so actually the amplitude of the current is highest in the middle and zero on the ends. This spatially and temporally varying current is the source of a spatially and temporally varying magnetic field. Similarly with charge and the e-fields, except that the amplitude of the charge is highest on the ends.
  4. Mar 18, 2010 #3
    Pretty much all waves are caused by an oscillation between kinetic energy and potential energy. There is a storage of energy in one form and release in another that are complimentary and this implies some sort of spring (as a crude physical analogy). Or actually 2 springs operating in tandem.

    The loading of the spring in the case of light is the build up (compression in terms of volume) of localized electric energy and then its release. Concurrently there is a release of magnetic energy followed by a build up in a way that both lag each by 90 degrees (or minus 90 degrees) when plotted over time.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook