What is meant by the term “critical frequency” at a given incidence angle?
The highest frequency which is refracted back to Earth✓Bonne réponse
The lowest frequency which is refracted back to Earth
The frequency at which the signal-to-noise ratio approaches unity
The frequency at which the signal-to-noise ratio is 6 dB
Explication
The critical frequency is the highest frequency a given layer will still bend back to Earth; above it the wave penetrates the layer and continues upward. Signal-to-noise ratio has nothing to do with it, which rules out the two SNR choices.