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.

Huygen's Principle - Reflection