Identify the correct statements from the following descriptions of various properties of electromagnetic waves.
(A) In a plane electromagnetic wave electric field and magnetic field must be perpendicular to each other and direction of propagation of wave should be along electric field or magnetic field.
(B) The energy in electromagnetic wave is divided equally between electric and magnetic fields.
(C) Both electric field and magnetic field are parallel to each other and perpendicular to the direction of propagation of wave.
(D) The electric field, magnetic field and direction of propagation of wave must be perpendicular to each other.
(E) The ratio of amplitude of magnetic field to the amplitude of electric field is equal to speed of light.
Choose the most appropriate answer from the options given below :

Solution

Physics sir · 4 steps

Picture E, B and the direction of travel as three mutually perpendicular axes, then test each statement.

1. Directions. In a plane EM wave, E⃗ and B⃗ are perpendicular to each other and both are perpendicular to the direction of travel, which is along E⃗ × B⃗. So (D) is correct, while (A) wrongly sends the wave along E⃗ or B⃗ and (C) wrongly makes the fields parallel.
2. Energy sharing. The electric energy density 1/2ε₀ E² equals the magnetic energy density B²/2μ₀ at every point, because B = E/c and c² = 1/(μ₀ ε₀). So (B) is correct.

   E₀/B₀ = c

3. Amplitude ratio. It is the electric amplitude divided by the magnetic amplitude that equals c. The ratio B₀/E₀ is 1/c, so (E) is wrong.
4. So. Only (B) and (D) are correct, which is option (b).

The idea to keep. In an EM wave, E, B and the direction of travel form a right-handed perpendicular set, the two fields share the energy equally, and E₀ = cB₀, never B₀ = cE₀.

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