When light passes through different mediums, such as air into glass, its speed changes due to the change in the refractive index of the medium. The refractive index is a property of the medium that describes how much light slows down or speeds up as it enters that medium.
When light enters a denser medium, like glass, its speed decreases because the refractive index of glass is higher than that of air. This decrease in speed is accompanied by a decrease in wavelength and a change in direction, known as refraction. Energy, on the other hand, is related to the frequency of light, which remains constant as it passes through different mediums.
So, when light slows down in a denser medium like glass, its wavelength decreases, but the energy associated with each individual photon remains the same. This means that the energy of the light does not change as it enters the glass.
When light exits the glass and returns to a less dense medium like air, it speeds up again and returns to its original speed and wavelength. The energy of the light remains constant throughout this process.
The apparent change in speed of light when passing through different mediums is due to the interaction of light with the electric fields of the atoms or molecules in the medium, which affects its propagation. However, the energy of the light itself does not change during these transitions.