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I found myself a little bit lost in those concepts. Assume there are many linearly polarized plane waves, all with equal amplitudes, all traveling along z direction, and their polarizations are uniformly distributed in xy plane. Then by superposition principle amplitude of total electric and magnetic fields is zero - there are as many waves polarized in one direction as there are polarized in the opposite direction. But the net intensity and energy is not zero, right? Since intensity and energy are proportional to the square of the amplitude. And so linear momentum carried by the waves is also not zero. If all those waves strikes some surface lying in the xy plane and are absorbed in it or are reflected by it, by the conservation of linear momentum it seems that the surface must gain some momentum.

If I took this picture of radiation pressure:

when struck by plane wave, an electron in the surface moves in the direction opposite to that of electric field of the wave, then there is a magnetic force (v cross B) along the z direction.

I end up confused, because when the surface is hit by an unpolarized light, electric field is zero, so the electron does not move and there is no force on the surface, and no radiation pressure.

I hope I've made my problem clear and thanks in advance for any comment.

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# Radiation pressure and unpolarized light

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