The truth is that the energy that was light has become the energy of an electron is an excited state. When electron emits the energy to return to the “ground state”, the energy is in the form of light. The light did not exist during this time! So, the velocity of the light never really decreases.
The light just has to “stop” existing for a short periods of time, because of the particles with which it interacts! So, the average velocity of light in the medium is always less than the velocity of light in a vacuum.
The index of refraction = (velocity of light in a vacuum) ÷ (average velocity of light in the medium)
The average velocity of light in the medium is always less than the velocity of light in a vacuum, so the index of refraction is always greater than 1.
All the effects of the index of refraction are determined by the structure and composition of the medium!
I hope this helps you understand how the index of refraction is related to interaction of light with matter!