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For an electromagnetic wave in vacuum, the fields E and B are mutually perpendicular. If the direction of polarization is X and the propagation direction is k, which relations hold?

1. X parallel to B, and k parallel to (E x B)
2. X parallel to E, and k parallel to (B x E)
3. X parallel to B, and k parallel to (B x E)
4. X parallel to E, and k parallel to (E x B)

Correct answer: X parallel to E, and k parallel to (E x B)

Solution

By convention the plane (and direction) of polarization is taken along the electric field E, so X is parallel to E. The wave travels along the Poynting vector S = (1/mu0) E x B, so k is parallel to E x B.

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