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.
In the picture above, wavelength is the distance from crest to crest (or trough to trough as well) along the length of the wave. The amplitude is the distance up and down from crest to trough. In classical wave physics the energy of a wave is related to its amplitude. The greater the amplitude, the greater the energy. When the waves hit the beach with 4 foot crests, they are more powerful than waves with 2 foot crests.