When light enters a different medium, such as air to glass, it indeed slows down due to interactions with the atoms or molecules in that medium. This change in speed is related to the refractive index of the material, which is a measure of how much the speed of light is reduced as it passes through the medium.
When light travels through a medium, it interacts with the electric fields of the atoms or molecules within that medium. These interactions cause the light to be absorbed and re-emitted by the atoms or molecules, effectively slowing it down. This absorption and re-emission process happens very quickly and at a microscopic level.
When the light reaches the interface between the two mediums, in this case, the interface between the glass and the air, it undergoes another interaction with the atoms or molecules at that boundary. This interaction causes the light to be re-emitted and allows it to continue its journey through the air.
Importantly, the speed of light in a given medium is determined by the properties of that medium and the nature of the interactions between light and matter within it. The energy of the light, on the other hand, is related to its frequency or wavelength. When light travels from one medium to another, its frequency remains the same, but its speed changes.
To clarify, when light slows down as it enters a medium, it does not gain or lose energy in the process. Instead, the energy carried by the light remains constant, but the speed at which it travels adjusts based on the properties of the medium it is passing through.
It's also worth noting that the change in speed of light when it passes through different mediums is responsible for phenomena such as refraction, which is the bending of light as it transitions from one medium to another. This bending is due to the change in speed of light, and it is what allows lenses and prisms to manipulate and focus light in various ways.