*Question E3A04:
In what direction does an electromagnetic wave travel?*  

*Correct Answer is  D: It travels at a right angle to the electric and magnetic fields*  

### AI Response: (may contain errors)  

In an electromagnetic wave, the electric field ($\mathbf{E}$) and the magnetic field ($\mathbf{B}$) are perpendicular to each other and to the direction of wave propagation. This is a fundamental property of electromagnetic waves, as described by Maxwell's equations.

### Explanation:

1. **Correct Answer: D - It travels at a right angle to the electric and magnetic fields**

   - **Why it's correct:**  
     Electromagnetic waves propagate in a direction that is perpendicular to both the electric field and the magnetic field. This is often visualized as the electric field and magnetic field oscillating in two perpendicular planes, with the wave traveling in a direction that is perpendicular to both of these planes. This is a characteristic of transverse waves.

2. **Why the other choices are incorrect:**

   - **Choice A: It depends on the phase angle of the magnetic field**  
     - Incorrect because the direction of wave travel is not determined by the phase angle of the magnetic field. The phase angle affects the timing of the wave's oscillations, not its direction.

   - **Choice B: It travels parallel to the electric and magnetic fields**  
     - Incorrect because electromagnetic waves are transverse waves, meaning the fields are perpendicular to the direction of wave travel, not parallel.

   - **Choice C: It depends on the phase angle of the electric field**  
     - Incorrect for the same reason as Choice A. The phase angle affects the oscillation timing, not the direction of travel.

### Useful Rule of Thumb:

- **Right-Hand Rule for Electromagnetic Waves:**  
  If you point your thumb in the direction of wave propagation, your index finger in the direction of the electric field, and your middle finger in the direction of the magnetic field, all three will be perpendicular to each other, forming a right-handed coordinate system.

This understanding is crucial for visualizing how electromagnetic waves propagate through space.