Slovník pojmůPhase velocity, group velocityPhase velocity determines the change of phase of wave per second. Phase velocity is not associated with the velocity of energy transmission, and therefore, it can be higher than velocity of light in vacuum c = 3 * 10+8 m/s. Phase velocity tends to infinity in all directions lying on the equiphase surface (i.e. on the surface associating points of the same phase): the phase changes for 2π radians within infinitely short time period here. On the other hand, group velocity determines the speed of energy transmission. Since in the directions, which lie on the wave-surface, energy is not transmitted (wave-surface is perpendicular to the propagation direction), group velocity is zero here. In common media, phase velocity and group one are mutually related according to Here vf denotes phase velocity, vg is group velocity, and v is free-space velocity of light. Zpět
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