*Question G3C02:
What is meant by the term “critical frequency” at a given incidence angle?*  

*Correct Answer is  A: The highest frequency which is refracted back to Earth*  

### AI Response: (may contain errors)  

The term "critical frequency" refers to the highest frequency that can be refracted back to Earth by the ionosphere at a vertical incidence. This is important for understanding how radio waves propagate over long distances.

1. **Correct Answer (A):**  
   - The critical frequency is the highest frequency that can be refracted back to Earth by the ionosphere when the radio wave is transmitted vertically. If the frequency is higher than the critical frequency, the wave will pass through the ionosphere and not be refracted back to Earth.

2. **Why Other Choices Are Incorrect:**
   - **B: The lowest frequency which is refracted back to Earth**  
     - This is incorrect because lower frequencies are generally refracted more easily by the ionosphere. The critical frequency is concerned with the highest frequency that can still be refracted.
   - **C: The frequency at which the signal-to-noise ratio approaches unity**  
     - This is unrelated to the concept of critical frequency. Signal-to-noise ratio deals with the quality of the received signal, not the refractive properties of the ionosphere.
   - **D: The frequency at which the signal-to-noise ratio is 6 dB**  
     - Like choice C, this is about signal quality rather than the refractive capability of the ionosphere.

3. **Useful Formulas or Rules of Thumb:**
   - The critical frequency ($f_c$) can be calculated using the formula:  
     $$ f_c = 9 \sqrt{N_{max}} $$  
     where $N_{max}$ is the maximum electron density in the ionosphere (in electrons per cubic meter). This formula shows that the critical frequency depends on the ionospheric conditions, particularly the electron density.

Understanding the critical frequency helps in determining the maximum usable frequency (MUF) for radio communication, which is crucial for effective long-distance communication.