The phenomenon of light slowing down as it passes through a material like glass or air is one of the most fascinating areas of physics, involving a complex interaction between light and materials. There are three main ways to look at this phenomenon, each of which provides a different perspective on the underlying physics.
View No. 1: It's all waves
The first perspective is based on the wave nature of light. When light waves encounter a material, they interact with the charged particles in the material. These charged particles can absorb and re-emit the light waves, which delays the overall progress of the light. This delay is what causes light to slow down in materials.
View No. 2: It's all particles
A second perspective on light slowing down is based on the particle nature of light. In this view, light is made up of individual photons, which can interact with the charged particles in materials. When a photon interacts with a charged particle, it can be absorbed and then re-emitted. This process also delays the progress of the photon, which causes light to slow down in materials.
View No. 3: It's all polaritons
A third perspective on light slowing down is based on the concept of polaritons. A polariton is a hybrid particle that is part photon and part phonon (a quantum of vibration). When light enters a material, it can interact with the phonons in the material to create polaritons. These polaritons then travel through the material at a slower speed than light.
Each of these three perspectives provides a different way of understanding how light slows down in materials. There is no one "correct" perspective, but each one can be useful in different contexts.