Topic: Electromagnetics
As the frequency approaches to cut-off frequency of wave guide, the correct statements is
- A. The phase velocity of waves tends to zero
- B. The phase velocity of waves tends to velocity of light
- C. The phase velocity of waves tends to infinite
- D. None of these
Answer: C. The phase velocity of waves tends to infinite
Explanation: Phase velocity, $v_{p}=\frac{c}{\sqrt{1-(\frac{f_{c}}{f_{0}})^{2}}}$. As the wave frequency $f_{o}$ approaches to cut-off frequency $f_{c}$, then $\frac{f_{c}}{f_{0}}$ reaches to 1. Therefore $1-(\frac{f_{c}}{f_{0}})^{2} \approx 0$. So, $v_{p}=\frac{c}{0} \rightarrow \infty$.