An electromagnetic wave is transporting energy in the negative direction. At a certain point and certain time the direction of electric field of the wave is along positive direction. What will be the direction of the magnetic field of the wave at that point and instant?
- A.Positive direction of
- B.Negative direction of
- C.Positive direction of
- D.Negative direction of
Show answer and solution
Correct option: A
Solution
For an EM wave, the energy (Poynting vector S = E × B/μ₀) points in the direction of propagation, which is negative z here. With E along positive y, we need E × B to point in the negative z direction. Using the right-hand rule: ŷ × x̂ = −ẑ, so B must be along positive x. This gives S ∝ ŷ × x̂ = −ẑ, consistent with propagation in the negative z direction.
[JEE Main 2023 · 25 Jan · Shift 1]