Structure of an electromagnetic wave — fields, orientation, and polarisation Pick the comprehensive correct statement set about the composition of EM waves, field orientation, and the meaning of polarisation.
-
AAn electromagnetic wave is composed of electric and magnetic fields
-
BThe electric and magnetic fields remain mutually orthogonal and transverse to the propagation direction
-
CIf an EM wave propagates in the z-direction, its electric vector lies in the XY plane along some direction
-
DPolarisation defines the orientation of the fields
-
EAll of these
Answer
Correct Answer: All of these
Explanation
Introduction / Context:Maxwell’s equations predict coupled electric (E) and magnetic (H) fields forming a transverse wave. Understanding their geometry and polarisation is fundamental in optics, antennas, radar, and remote sensing calibration.
Given Data / Assumptions:
- Homogeneous, source-free medium.
- Plane wave approximation.
- Right-handed E–H–k triad.
Concept / Approach:In free space, E ⟂ H ⟂ k (propagation direction). For propagation along z, E and H lie in the XY plane. Polarisation specifies how the E-field vector is oriented (and evolves) in time and space (linear, circular, elliptical).
Step-by-Step Reasoning:
1) Composition: EM waves comprise E and H fields.2) Orthogonality: E ⟂ H and both are transverse to k.3) Directional example: k along z ⇒ E in XY; H also in XY and orthogonal to E.4) Polarisation: orientation/trajectory of E defines wave polarisation state.Verification / Alternative check:Antenna polarisations (H/V, RHCP/LHCP) demonstrate practical importance for link budgets and scattering behaviour.
Why Other Options Are Wrong:
- Each individual statement is correct; the comprehensive choice is appropriate.
Common Pitfalls:Assuming longitudinal components exist in free space plane waves; mixing up polarisation of E vs H (E defines polarisation by convention).
Final Answer:All of these