If we consider a source, radiating electromagnetic radiation in all directions, intensity falls off as 1/r^2, and thus electric field amplitude falls off as 1/r. But if I consider exactly one wave emanating from the source, there is no way that that one day can lose energy and therefore it's amplitude of electric field has to be a constant. Whats wrong with my analysis


1 Answer 1


The energy is spread out over a larger surface area as the wave expands. The electric field squared is proportional to the energy density, not the energy. So as the wave expands, the energy per unit area goes down, but the area goes up, in a way that keeps the total energy of the wave fixed.

  • $\begingroup$ If electric field square is proportional energy density, that means it will decrease with increasing in distance , as energy density decreases with distance. But for a lone wave the electric field is a constant right $\endgroup$ Feb 27, 2018 at 2:43
  • $\begingroup$ Only if by "lone wave" you mean "plane wave." If the wave is spreading out at all, the electric field is not constant. $\endgroup$
    – Chris
    Feb 27, 2018 at 3:01

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