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An electromagnetic wave is transporting energy in the negative z direction. At a certain point and certain time the direction of electric field of the wave is along positive y direction. What will be the direction of the magnetic field of the wave at that point and instant?

Asked in JEE Main 25th Jan 1st Shift 2023 · Direction from E cross B, and oblique propagation

• (1) Positive direction of z.
• (2) Negative direction of y.
• (3) Positive direction of x.
• (4) Negative direction of x.

Step-by-step solution

Why the other options are wrong

• (1) comes from z is the propagation direction; B must be perpendicular to it
• (2) - y is antiparallel to E, and E × B would vanish
• (4) B̂ = -ı̂ gives ȷ̂ × (-ı̂) = +k̂, energy along +z, the wrong way

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