Which of the following statements is true about electromagnetic waves propagating in air?

• A. The electric $E$ and magnetic $B$ fields are perpendicular to each other and to the direction of propagation.
• B. Only the electric field oscillates; the magnetic field remains constant.
• C. Electromagnetic waves in air require a medium with free charges to propagate.
• D. The electric and magnetic fields are parallel to the direction of propagation.

Answer: (A)

Verified step by step guidance

1. Recall the fundamental nature of electromagnetic (EM) waves: they consist of oscillating electric and magnetic fields that propagate through space.
2. Understand that in an EM wave, the electric field \( \mathbf{E} \) and magnetic field \( \mathbf{B} \) are always perpendicular to each other.
3. Recognize that both \( \mathbf{E} \) and \( \mathbf{B} \) fields are also perpendicular to the direction of wave propagation, forming a mutually orthogonal set of vectors.
4. Note that electromagnetic waves do not require a medium with free charges to propagate; they can travel through a vacuum (or air) because they are self-sustaining oscillations of electric and magnetic fields.
5. Conclude that the correct statement is: 'The electric \( \mathbf{E} \) and magnetic \( \mathbf{B} \) fields are perpendicular to each other and to the direction of propagation.'