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 distance between wavefronts is the wavelength. All observers will hear the same frequency, which will be equal to the actual frequency of the source. For a movie showing how circular waves can be created (in terms of particle motion and wave motion) go here.