Fresnel diffraction occurs when a wavefront encounters an obstacle or aperture, and the wavefront is at a finite distance from the aperture, resulting in a complex interference pattern. It is characterized by the near-field regime where the wavefront curvature is significant, differing from Fraunhofer diffraction, which assumes parallel wavefronts at a large distance.
Phase velocity is the speed at which the phase of a wave propagates through space, which can differ from the actual speed of energy or information transmission in a medium. It is calculated as the frequency of the wave divided by its wavenumber and is crucial in understanding wave phenomena such as dispersion.