When light passes from one medium to another, such as from air to glass, it can indeed slow down due to the change in the refractive index of the materials. However, it's important to note that the speed of light is determined by the properties of the medium it is traveling through, and not by the energy it carries.
When light enters a denser medium like glass, its velocity decreases because the interactions between the photons and the atoms or molecules in the material cause a delay in their propagation. This delay is reflected in a reduction in the speed of light. However, the energy of the photons themselves remains unchanged during this process.
As light exits the glass and returns to air or another less dense medium, it speeds up again because the properties of the new medium allow for a higher velocity. The energy of the photons remains constant throughout this transition. The change in speed is a result of the different interaction between the photons and the atoms or molecules in each medium.
It's worth noting that the concept of energy conservation still holds true in this scenario. The total energy of the system is conserved as the light passes through different media, but the speed of light can vary due to the refractive index and the interaction with the atoms or molecules in those media.