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Q & A: energy nodes in propagating wave

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Most recent answer: 02/06/2014
Q:
If the light is propagating in z-direction, its electric field component oscillates in x-direction and magnetic field in y-direction. Interestingly, in z-direction, there comes a point (node) where both electric and magnetic fields are zero. (please refer: http://mysite.du.edu/~lconyers/SERDP/em_electric_magnetic_propagating_waves.jpg) Since both electric and magnetic fields are zero after a regular interval, I was wondering if energy should be zero at such points which subsequently implies that energy is not conserved in Electromagnetic wave propagation? How to resolve this?
- Vishvendra Poonia (age 22)
Mumbai
A:

Yes, in a linearly polarized E-M wave there are points (sheets, actually) where both E and B are zero, so the energy density is zero. Those points, however, move forward at the speed of light, in the z direction in your example. So the energy contained in the fields in between those nodes also moves forward at the speed of light. The flow is described by the Poynting vector.()

Mike W.


(published on 02/06/2014)

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