File talk:Light-wave.png

From the page of the image: The picture is wrong. The phase between the Electric and the Magnetic fields should be 90 degree and not zero degree, as shown on this picture.

Regards,

Boris Spasov, bspasov@yahoo.com

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The picture is not wrong, the phase is really 0 degree. -- Harp 11:59, 18 Mar 2005 (UTC)

Yes, the phase is 0 degrees in a plane wave through empty space. The nearest reference that comes to hand Fundamentals of Physics by David Halliday and Robert Resnick (1988) has the same illustration in Figure 10 of Chapter 38: Electromagnetic waves. (Also EM Waves and Polarization http://www.newton.dep.anl.gov/askasci/phy00/phy00783.htm and electromagnetic wave http://www.physlink.com/Education/AskExperts/ae436.cfm )

But I think it would be good if the light article went into more explanation.

Near some pieces of equipment, the E field and the H field really are 90 degrees out of phase. Other pieces of equipment generate E fields more-or-less independent of H fields, and vice versa.

Perhaps you are thinking of the 90 degree phase difference in a LC "[tank circuit]" http://www.allaboutcircuits.com/vol_2/chpt_6/1.html . Other situations where they are 90 degrees out of phase: "Since the current and voltage that produce these E and H fields are 90 degrees out of phase, the fields will also be 90 degrees out of phase." http://www.tpub.com/content/neets/14182/css/14182_63.htm "the magnetic (H) field and the electric (E) field plotted against time. Note that the two fields are 90 degrees out of phase with each other." http://www.tpub.com/content/neets/14182/css/14182_52.htm

--DavidCary 08:10, 26 Jun 2005 (UTC)

Just a suggestion: if the "distance", as captioned in the picture, is going to the right,i.e. the EM wave is travelling to the right, should the magnetic field be in reversed position? in another word, when the electric field is rising to positive y-direction, should the magnetic field rising to the positive z-direction instead of the negative z-direction as in the picture? i just think of the Poynting Vector, whose direction is E cross B:the one in which EM wave travels.

Here are the Maxwell’s equations for the vacuum,

Nabla-operator Dot-multiplied with E-vector = 0 Nabla-operator Dot-multiplied with H-vector = 0 Nabla-operator Cross-multiplied with E-vector = - mu-not.dH/dt Nabla-operator Cross-multiplied with H-vector = + epsilon-not.dE/dt

When on your chart the H-field reaches extreme value (maximum or minimum), its derivative in time becomes equal to ZERO (the tangent to the curve in time is horizontal). So, the value of the corresponding E-field on your chart should be equal to Zero also, and it is not (see the equation above). Therefore the phase between E and H can not be ZERO, as it is shown on your picture. This argument is E/H symmetric.

In other words, according to Maxwell’s equation, when on your chart E=max or E=min, the corresponding to it H-value should be equal to zero (and vice versa).

Regards, Boris Spasov.

to: Just a suggestion: if the "distance", as captioned in the picture, is going to the right,i.e. the EM wave is travelling to the right, should the magnetic field be in reversed position? in another word, when the electric field is rising to positive y-direction, should the magnetic field rising to the positive z-direction instead of the negative z-direction as in the picture?

since ExB forms a right handed system you are right. i also wondered why the magnetic component is in the wrong phase/ wrong z direction if the wave travells to the right the B field should point out of the sketch and not into

Perk

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