ANS:B - polarization

Answer - D.

1. The concept of phase velocity is valid if the harmonic wave is propagated with constant wave form; this condition is always satisfied in linear media.

2. Velocity dispersion occurs if the phase velocity is dependent on frequency or, stated differently, on wavelength.

3. In the absence of dispersion any wave will propagate, without changing its wave form, at a velocity equal to the phase velocity.

4. If dispersion occurs, non-harmonic waves change their wave form, and the conventional concept of velocity no longer applies.