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.
A wave is a disturbance that travels through a medium in a periodic fashion, carrying energy without transporting matter. Waves are described and measured by five wave parameters: the period, the frequency, the amplitude, the wavelength, and the speed. The period of a wave is the time it takes to complete one cycle.