Calcite, the most common form of natural calcium carbonate (CaCO3), a widely distributed mineral known for the beautiful development and great variety of its crystals.
A wavefront is a surface over which an optical wave has a constant phase. For example, a wavefront could be the surface over which the wave has a maximum (the crest of a water wave, for example) or a minimum (the trough of the same wave) value. The shape of a wavefront is usually determined by the geometry of the source.
The wave propagation speed is the speed by which the wavefront of the wave passes through a medium. The wavefront is the locus of the same phase point in the wave. It is also known as Velocity factor (VF) . The velocity factor has different equation for different medium.. Group Velocity: It is the velocity with which the amplitude of the wave travels in the medium. It is given by the formula; $V_{G}$=$\frac{d\omega}{dk}$. So, it can also be defined as the rate of change of the angular frequency of the wave with respect to the angular wave number of the wave.