*Question T3B01:
What is the relationship between the electric and magnetic fields of an electromagnetic wave?*  

*Correct Answer is  D: They are at right angles*  

### AI Response: (may contain errors)  

The correct answer to the question about the relationship between the electric and magnetic fields of an electromagnetic wave is:

**D: They are at right angles.**

### Explanation of the Correct Answer:
- In an electromagnetic wave, the electric field ($E$) and the magnetic field ($B$) are always perpendicular to each other and to the direction of wave propagation. This relationship can be visualized as follows:
  - If the wave is traveling in the $z$ direction, the electric field might oscillate in the $x$ direction, while the magnetic field oscillates in the $y$ direction. This can be represented as:
  
  $$E \perp B \perp \text{direction of propagation}$$

### Why the Other Choices are Incorrect:
- **A: They travel at different speeds**  
  This is incorrect because both the electric and magnetic fields in an electromagnetic wave travel at the same speed, which is the speed of light in a vacuum, denoted as $c \approx 3 \times 10^8 \text{ m/s}$.

- **B: They are in parallel**  
  This is incorrect because if the electric and magnetic fields were parallel, they would not be able to propagate as a wave. They must be perpendicular to each other to create the transverse wave nature of electromagnetic radiation.

- **C: They revolve in opposite directions**  
  This is incorrect as well. The electric and magnetic fields do not revolve; they oscillate sinusoidally in their respective planes. The concept of "revolving" does not apply to the behavior of these fields in an electromagnetic wave.

### Useful Rules of Thumb:
- Remember that electromagnetic waves are transverse waves, meaning the oscillations of the fields are perpendicular to the direction of wave travel.
- The relationship can be summarized with the right-hand rule: if you point your thumb in the direction of wave propagation, your index finger in the direction of the electric field, your middle finger will point in the direction of the magnetic field. 

This understanding is crucial for grasping the fundamental principles of electromagnetism and is a key concept for the Extra Class exam.