Physics · Electromagnetic Waves · NEET
It is always E x B. The velocity (Poynting) direction v is along E x B, in that order. If you flip to B x E you get the exact opposite direction and lose the mark. Fix the order in your head as E first, then B: E cross B gives the way the wave moves.
B is along +z. You need E x B to point along +x (the velocity). Check unit vectors: j x k = i, which is +x. Since E is +y (j) and this works with B = +z (k), the answer is +z. If you had guessed -z, then j x (-k) = -i = -x, which is wrong. This is the exact NEET 2018 question.
B points along v x E. Start from v = E x B. Cross both sides with E on the left and use the perpendicular geometry: since E, B, v are mutually perpendicular, the shortcut is B is parallel to v x E. Quick check with i, j: if v is +x (i) and E is +y (j), then v x E = i x j = k = +z, matching B = +z.
Because EM waves are transverse. NCERT states E and B are perpendicular to each other and both perpendicular to the direction of propagation. So all three (E, B, v) form a right-handed set at 90 degrees to one another. B can never lie along E or along v.
Yes, the sign of the phase tells you the travel direction. A (kx - wt) wave moves along +x; a (kx + wt) wave moves along -x. First fix v from the phase sign, then apply v = E x B. In NEET 2025 the phase (5x + 1.5e9 t) meant the wave moved along -x, and only after fixing that could you get the correct B direction.
An EM wave is propagating in a medium with velocity v = v i (along +x). The instantaneous oscillating electric field of this EM wave is along the +y axis. Then the direction of the oscillating magnetic field of the EM wave will be along:
For a plane electromagnetic wave propagating in the +x-direction, which one of the following combinations gives a correct possible pair of directions for the electric field (E) and magnetic field (B) respectively?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Use B along v x E. Since v = E x B, rearranging for perpendicular vectors gives B parallel to v x E. Plug in unit vectors i, j, k and read off the answer.
Yes. EM waves are transverse, so E is perpendicular to B, and both are perpendicular to the propagation direction v. They form a right-handed set.
Look at the phase sign. A term like (kx - wt) means the wave travels along +x. A term like (kx + wt) means it travels along -x. Fix v from this before applying E x B.
No. Whether in vacuum or in a medium, v is still along E x B and the three vectors stay mutually perpendicular. Only the speed changes, not the direction relationship.
Check two things: E . B = 0 (perpendicular) and E x B along the given travel direction. This is exactly how NEET 2021 tested it - only the pair passing both tests is correct.
