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Question:

An electromagnetic wave in vacuum has the electric and magnetic field E and B, which are always perpendicular to each other. If the direction of polarization is given by X and that of wave propagation by k, then:

X || E and k || E × B

X || B and k || B × E

X || B and k || E × B

X || E and k || B × E

Solution:

Direction of polarization is parallel to electric field, ∴ X || E
And direction of wave propagation is parallel to E × B
k || E × B