What happens to light when it passes through substances of different densities?

When light travels from one medium to another, such as from air to water, it experiences changes due to the different refractive indices of the substances. As light passes through media of varying densities, its speed is affected; light travels slower in denser substances than in less dense ones. This change in speed causes light to also change direction, a phenomenon known as refraction.

In more detail, when light moves into a denser medium, like going from air (less dense) into water (more dense), it slows down and bends towards the normal line—a line that is perpendicular to the surface at the point of incidence. Conversely, when light exits a denser medium into a less dense one, it speeds up and bends away from the normal.

Both alterations in speed and the resulting change in direction are critical concepts in understanding how light interacts with various materials, which is why the combination of changing speed and direction is the most comprehensive and accurate choice in this context.