\prob{79}
The dispersion curve shown above relates the angular frequency to the wave number k. For waves with wave numbers lying in the range $k_1
 They are in opposite directions.
 They are in the same direction and the phase velocity is larger
 They are in the same direction and the group velocity is larger
 The phase velocity is infinite and the group velocity is finite.
 They are the same in direction and magnitude

Wave Phenomena}Group Velocity
Recall that the group velocity is given by and the phase velocity is given by .
In the region between and , the derivative is a constant negative quantity (approximately just the derivative of a line with negative slope). However, is positive in this region. Thus, the phase and group velocity are traveling in opposite directions. Thus, choose choice (A).
