What happens to light when it travels from air to glass?

When light travels from air into glass, it slows down due to the difference in the refractive index between the two media. Air has a lower refractive index compared to glass, which means that light moves more quickly in air than it does in glass. The refractive index is a measure of how much the speed of light is reduced when it enters a material; therefore, when light enters glass, its speed decreases.

This reduction in speed also causes the light to change direction slightly, an effect known as refraction. Refraction occurs because the light waves have to adjust to the new speed in the denser medium (glass). The slowing down of light is a fundamental concept in optics and is crucial for understanding how lenses and other optical devices operate.