An electromagnetic wave in vacuum propagates along +z. Its electric-field x-component is Eₓ = E₀ cos(kz - ωt). The corresponding magnetic field is:
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Question

An electromagnetic wave in vacuum propagates along +z. Its electric-field x-component is Eₓ = E₀ cos(kz − ωt). The corresponding magnetic field is:

G1 Physics · Accepted Answer

For a plane electromagnetic wave in vacuum, E₀/B₀ = c, so B₀ = E₀/c. The direction of propagation is along E × B. The wave travels along +z and E is along +x. Therefore B must be along +y because (+x) × (+y) = +z. The electric and magnetic fields are in phase, so the magnetic field has the same phase factor: B magnitude = (E₀/c) cos(kz − ωt). Thus B is along +y. Correct option: A.