We learn that EM waves cause the electrons in a conductor to move around. For example, air to ground radar shows the ocean as having few returns compared to land. Water molecules absorb the energy. Doesn't the induced motion of electrons in the metal in chaotic eddy currents dissipate the energy? I read this:


but was not enlightened.


1 Answer 1


Because metals are electrically conductive, an incoming radar pulse induces a current to flow in the surface of the metal. That current flow then radiates a replica of the original wave, moving in the opposite direction.

  • $\begingroup$ And (mostly) cancels out the forward propagating wave. $\endgroup$
    – ProfRob
    Oct 12, 2021 at 7:32
  • $\begingroup$ I have two issues with that explanation: 1) when an em emitter is turned on, it generates waves at all possible phases and in all directions. Certainly we collimate that emission to one direction but what arrives at the metal at any given instant are em waves in all possible phases. And at any given instant there are two waves that are half a wavelength out of phase and should cancel out (the eddy currents in the metal go in opposite directions). So no em wave should be generated. $\endgroup$ Oct 17, 2021 at 13:10
  • $\begingroup$ 2) Planck's explanation of black body radiation used a cavity that was electrically conductive. He stated that the E field at the surface of the cavity was zero. He used this fact him to calculate the standing waves in the box were integral multiples of some base frequency. But if the E field in the metal is zero, how does the metal get any energy to emit a reflected wave? $\endgroup$ Oct 17, 2021 at 13:14
  • $\begingroup$ @aquagremlin, a radar emitter is 1) unidirectional (it is a collimated beam) and 2) it is almost monochromatic (one frequency). for plane waves (far from the source, as is the case for radar) none of your objections in your first comment apply. regarding your second comment, an EM wave impinging on an electrically conductive object will induce a current loop that resides in the skin of the object (almost no penetration depth). This fact allows straight pieces of thin wire to act as receiving antennas for radio frequency EM waves since the wire is "all skin". $\endgroup$ Oct 17, 2021 at 21:46
  • $\begingroup$ thank you for continuing the discussion. Regarding monochromatic emission, this does not mean that all the waves are in phase. Even if the eddy currents occur only on the skin of the object, the electrons will circulate in one direction as the E field increases, but as the trough of the EM wave approaches and the E field is decreasing, then the electrons will circulate the other way. As there are differrent phased EM waves impinging on different electrons, the net result should be zero as they all cancel out. $\endgroup$ Oct 17, 2021 at 21:54

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