Wave velocity can be measured as phase velocity or group velocity. Phase velocity measures the speed of a single wave, while group velocity measures the speed of a wave packet. The relationship between phase and group velocities can vary, and can be affected by the properties of the medium and the frequency of the waves. The phase velocity of waves can also be measured in a vacuum.
When studying wave velocity, or the speed at which waves move, there are two distinct types that can be calculated, which are phase velocity and group velocity. Phase velocity measures the velocity of a single wave, while group velocity measures the velocity of a wave packet, or envelope. A wave packet is a group of waves that move as a single unit, and therefore can be measured as a single entity.
Speed is calculated by measuring the distance traveled at a given moment. Phase velocity measurement involves measuring the distance a wavelength travels over a period of time. A wavelength is the distance from the crest, or top, of one wave to the crest of the next wave. Phase velocities and group velocities do not have to be the same in a wave packet.
Several relationships between phase and group velocities can be observed. When the phase and group velocities are the same, each component wave moves with the whole packet or the waveform moves as a single unit. If the phase velocity equals the negative group velocity, the component waves move in the opposite direction of the wave packet.
Phase and group velocities can also be greater or less than each other. Waves with phase velocities greater than the group velocity will move faster than the envelope. The opposite is true when group velocity is higher. In this case, the component waves move slower than the full envelope.
In addition to the above scenarios, phase velocity or group velocity can be equal to zero. When this happens, the component waves or wave packet are stationary. With a phase velocity of zero, the wave packet moves on stationary component waves. A group velocity of zero means that the envelope is at rest and the component waves are moving through it.
Phase speed will be different for different types of media. This is because the properties of the medium play a role, so light waves, sound waves and waves in different liquids can react differently. Furthermore, the frequency of the waves can have a direct effect on the phase velocity, since different wave frequencies usually travel at different speeds. The phase velocity of waves can also be measured in a vacuum. This must be taken into consideration as the permeability and permittivity of the vacuum will have a direct effect on the phase and group velocities of the medium to be measured.
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