A Phase Shift of 180º Occurs When a Light Wave

A phase shift of 180º occurs when a light wave

A) is transmitted through a boundary surface into a medium that is more dense than the medium from which the wave came.
B) is transmitted through a boundary surface into a medium that is less dense than the medium from which the wave came.
C) reflects from the boundary surface of a medium that is less dense than the medium in which the wave is traveling.
D) reflects from the boundary surface of a medium that is denser than the medium in which the wave is traveling.
E) Both c and d are correct.