Question E3A05:
How are the component fields of an electromagnetic wave oriented?  

Choices:  
A: They are parallel  
B: They are tangential  
C: They are at right angles  
D: They are 90 degrees out of phase    

Correct Answer is  C: They are at right angles  

Correct answer: C — They are at right angles

Why C is correct:
- In a radiating (far-field) electromagnetic wave, the electric field vector $\vec E$ and magnetic field vector $\vec H$ are perpendicular to each other and both are perpendicular to the direction of propagation.
- The propagation direction is given by the Poynting vector:
  $$\vec S = \vec E \times \vec H$$
  This cross product implies $\vec E$ and $\vec H$ are at right angles.

Why the other choices are incorrect:
- A: Parallel — $\vec E$ and $\vec H$ are not parallel in a plane wave; they are orthogonal.
- B: Tangential — “Tangential” refers to boundary conditions at surfaces, not the mutual orientation of $\vec E$ and $\vec H$ in free space. The fields are not tangential to each other; they are perpendicular.
- D: 90 degrees out of phase — In the far field (and in lossless media), $\vec E$ and $\vec H$ are in phase; their peaks occur at the same time. A 90° phase difference is characteristic of reactive near-field components, not the radiating wave.

Useful rules of thumb and formulas:
- Orientation: Use the right-hand rule — point your index finger along $\vec E$, your middle finger along $\vec H$; your thumb points in the direction of propagation.
- Field ratio (intrinsic impedance of free space):
  $$\frac{E}{H} = \eta_0 \approx 377\ \Omega$$
- Both $\vec E$ and $\vec H$ are transverse to the direction of propagation (a TEM wave).