What Causes Refraction in Different Media?

[Claude Bile]

When waves change velocity, it is necessary for the wave direction (the wave-vector) to change direction as they refract across a boundary; so as to preserve field continuity across the boundary.

Microscopically, refraction occurs because atoms act as driven oscillators that impart a phase shift on the incident wave - ultimately causing the (group velocity of the) wave to slow.

Claude.

 

[sophiecentaur]

When waves change velocity, it is necessary for the wave direction (the wave-vector) to change direction as they refract across a boundary; so as to preserve field continuity across the boundary.

Microscopically, refraction occurs because atoms act as driven oscillators that impart a phase shift on the incident wave - ultimately causing the (group velocity of the) wave to slow.

Claude.

Yes: atoms, molecules, ions and plain free electrons, all in different circumstances and for different frequencies of EM.
A good non-QM description is always best when possible, I always say.