*Question E3A10:
What determines the speed of electromagnetic waves through a medium?*  

*Correct Answer is  D: The index of refraction*  

### AI Response: (may contain errors)  

The speed of electromagnetic waves through a medium is primarily determined by the index of refraction of that medium.

### Correct Answer: The index of refraction

- The index of refraction ($n$) is defined as the ratio of the speed of light in a vacuum ($c$) to the speed of light in the medium ($v$):
  
  $$
  n = \frac{c}{v}
  $$

- This means that the higher the index of refraction, the slower the electromagnetic waves travel through that medium. For example, in glass, the index of refraction is typically around 1.5, indicating that light travels slower in glass than in a vacuum.

### Why the other choices are incorrect:

- **A: Resistance and reactance**  
  - These terms relate to electrical circuits and impedance, not directly to the speed of electromagnetic waves in a medium. They affect how signals are transmitted but do not determine wave speed.

- **B: Evanescence**  
  - This term refers to evanescent waves, which occur in specific contexts like total internal reflection. It does not describe a general property that determines wave speed in a medium.

- **C: Birefringence**  
  - Birefringence is a property of certain materials where the index of refraction varies with polarization and direction of light. While it affects how light behaves in those materials, it does not determine the fundamental speed of electromagnetic waves in a medium.

### Summary

- The index of refraction is the key factor that determines the speed of electromagnetic waves in a medium. Understanding this concept is crucial for grasping how different materials affect wave propagation.