*Question T3C11:
Why is the radio horizon for VHF and UHF signals more distant than the visual horizon?*  

*Correct Answer is  C: The atmosphere refracts radio waves slightly*  

### AI Response: (may contain errors)  

The correct answer to why the radio horizon for VHF and UHF signals is more distant than the visual horizon is:

**C: The atmosphere refracts radio waves slightly.**

### Explanation of the Correct Answer:
- **Refraction of Radio Waves**: 
  - The atmosphere bends (refracts) radio waves due to changes in air density. This bending allows radio waves to travel beyond the geometric line of sight, extending the effective range of VHF and UHF signals.
  - The refractive index of the atmosphere is slightly greater than 1, which causes the waves to curve downwards towards the Earth, allowing them to reach further distances than what would be visible.

### Why the Other Choices are Incorrect:
- **A: Radio signals move somewhat faster than the speed of light**: 
  - This is incorrect because all electromagnetic waves, including radio signals, travel at the speed of light in a vacuum, which is approximately $3 \times 10^8$ m/s. They do not exceed this speed.

- **B: Radio waves are not blocked by dust particles**: 
  - While it is true that radio waves can penetrate some obstacles better than visible light, this does not explain the increased distance of the radio horizon. Dust particles can scatter radio waves, but they do not significantly affect the horizon distance.

- **D: Radio waves are blocked by dust particles**: 
  - This statement is misleading. While dust can attenuate signals, it does not block radio waves in the same way it can block visible light. The presence of dust does not contribute to the increased distance of the radio horizon.

### Useful Concepts:
- **Radio Horizon**: The distance to which radio waves can travel, which is generally greater than the visual horizon due to atmospheric refraction.
- **Visual Horizon**: The distance at which an observer can see, limited by the curvature of the Earth.

### Rule of Thumb:
- The radio horizon can be approximated using the formula:
$$
d \approx 1.414 \sqrt{h}
$$
where:
- $d$ = distance to the horizon in miles
- $h$ = height of the antenna in feet

This formula shows that increasing the height of the antenna increases the distance to the radio horizon, further enhanced by atmospheric refraction.