In a plane electromagnetic wave traveling in the +x direction in vacuum, which of the following is true?

In a plane electromagnetic wave traveling in the +x direction in vacuum, which of the following is true?

• A. E ⟂ B and |E| = c|B|
• B. E ∥ B
• C. |E| = B/c
• D. E, B, and k are not related

Answer: (A)

In vacuum, a plane electromagnetic wave has electric and magnetic fields that are perpendicular to each other and to the direction of travel, with their magnitudes linked by |E| = c|B|. If the wave moves along +x, E and B point in transverse directions (for example, E along y and B along z), and the fields are in phase so energy flows in the +x direction (E × B gives the Poynting vector along +x). The relation B = (1/c) k̂ × E formalizes this, showing E, B, and the propagation direction are all tied together. This makes the statement true. The idea that E and B are parallel is incompatible with a vacuum plane wave, the reversed ratio |E| = B/c is not correct for vacuum, and claiming E, B, and k are unrelated contradicts how plane waves propagate with E and B perpendicular to k.

In vacuum, a plane electromagnetic wave has electric and magnetic fields that are perpendicular to each other and to the direction of travel, with their magnitudes linked by |E| = c|B|. If the wave moves along +x, E and B point in transverse directions (for example, E along y and B along z), and the fields are in phase so energy flows in the +x direction (E × B gives the Poynting vector along +x). The relation B = (1/c) k̂ × E formalizes this, showing E, B, and the propagation direction are all tied together.

This makes the statement true. The idea that E and B are parallel is incompatible with a vacuum plane wave, the reversed ratio |E| = B/c is not correct for vacuum, and claiming E, B, and k are unrelated contradicts how plane waves propagate with E and B perpendicular to k.