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A plane electromagnetic wave travels along the +y direction. Which of the following pairs of electric-field and magnetic-field components is a valid combination for this wave?

1. Eₓ, B_z or E_z, Bₓ
2. E_y, B_y or E_z, B_z
3. E_y, Bₓ or Eₓ, B_y
4. Eₓ, B_y or E_y, Bₓ

Correct answer: Eₓ, B_z or E_z, Bₓ

Solution

An EM wave is transverse, so E and B both lie in the x-z plane (perpendicular to y) and are perpendicular to each other. Valid pairs are (Eₓ, B_z) and (E_z, Bₓ), since E cross B then points along the propagation direction y.

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