When light passes through a medium like glass and then emerges back into a vacuum or another medium, it resumes its previous speed due to a phenomenon called refraction. Refraction occurs because the speed of light changes when it travels from one medium to another with a different optical density.
When light enters a medium like glass, it interacts with the atoms or molecules of the material. These interactions cause the electric field of the light wave to induce oscillations in the charged particles of the medium. The charged particles then re-emit the light, which leads to a delay in the overall propagation of the wave. This delay results in a reduction in the speed of light within the medium compared to its speed in a vacuum.
The change in speed of light when it enters a medium is determined by the refractive index of the material. The refractive index is a property of the medium that describes how much the speed of light is reduced when it passes through that medium. Different materials have different refractive indices.
When light exits the medium and enters another medium or a vacuum, the opposite process occurs. The light wave interacts with the particles of the new medium, and the electric field of the wave induces oscillations in these particles. These oscillations re-emit the light, causing it to resume its previous speed as it returns to its original medium.
In summary, the phenomenon of refraction is responsible for the change in the speed of light when it enters and exits a medium, such as glass. The interactions between light and the particles of the medium cause the delay and subsequent resumption of its previous speed.