A metallic wave guide of square cross-section of side L is excited by an electromagnetic wave of wave-number k. The group velocity of the TE11 mode is
1
\(ckL/\sqrt {{k^2}{L^2} + {\pi ^2}} \)
2
\(\frac{c}{{kL}}\sqrt {{k^2}{L^2} - 2{\pi ^2}} \)
3
\(\frac{c}{{kL}}\sqrt {{k^2}{L^2} - \,\,{\pi ^2}} \)
4
\(kL/\sqrt {{k^2}{L^2} + \,2\,{\pi ^2}}\)