In an electromagnetic wave, at an instant and at particular position, the electric field is along the negative z-axis and magnetic field is along the positive x-axis. Then the direction of propagation of electromagnetic wave is:

Solution

Physics sir · 3 steps

The wave always travels along E⃗ × B⃗, so take that cross product carefully, sign included.

1. Direction rule. In an electromagnetic wave, E⃗, B⃗ and the direction of travel are mutually perpendicular. The wave moves along E⃗ × B⃗, the same direction as the Poynting vector.
2. Write the fields. E⃗ points along the negative z-axis, so its direction is -k̂. B⃗ points along the positive x-axis, so its direction is +î.

   (-k̂) × î = -(k̂ × î) = -ĵ

3. So. Using the cyclic order k̂ × î = ĵ, the product is -ĵ. The wave travels along the negative y-axis, option (c).

The idea to keep. For any EM wave, propagation is along E⃗ × B⃗. Write each field as a signed unit vector first, then use the cyclic order î → ĵ → k̂.

Answer from the NTA final answer key. Solution by PrepWiser.