Optics/Huygens’ Principle

thumb|left|200px|Huygen's Principle - Refraction Huygens' Principle recognizes that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves; additionally, the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed. This view of wave propagation helps better understand a variety of wave phenomena, such as diffraction. Fermat's Principle, the principle of least time, can be derived from it. thumb|500px|Huygen's Principle - Reflection