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 defined as a burst of emotions, a swell along the surface caused by wind, a sudden occurrence of something, or a back and forth motion with your hand. When you suddenly feel really sad, this is an example of a wave of sadness. The water movement that you use to go surfing in is an example of a wave.