Final answer:
When a beam of light moves from air to glass, it slows down due to the higher refractive index of glass. When it moves back into the air, its speed increases.
Step-by-step explanation:
When a beam of light travels from air to glass, it slows down because the refractive index of glass is greater than that of air. The refractive index determines how fast light travels through a medium. As a result, the speed of light decreases when it moves from air to glass.
On the other hand, when the light wave moves from the glass back into the air, it speeds up again. This is because the refractive index of glass is higher than that of air, causing the light to bend away from the normal. As a result, the light wave travels faster through air than through glass.
Therefore, when the light wave moves back into the air from the glass, its speed increases.
Learn more about Refraction of light