What phenomenon occurs when light passes through a boundary and changes direction?

When light passes through a boundary and changes direction, the phenomenon involved is refraction. This occurs when light travels from one medium to another, such as from air to water, leading to a change in its speed. The difference in the refractive index of the two mediums causes the light to bend. The degree of this bending can be described by Snell's Law, which relates the angles of incidence and refraction to the indices of refraction of the two materials.

In contrast, reflection involves light bouncing off a surface, maintaining its angle relative to the surface. Diffraction is the bending of light waves around obstacles or through openings, which occurs without the characteristic change in speed that defines refraction. Dispersion happens when different wavelengths of light refract at slightly different angles, such as when white light is separated into its constituent colors through a prism. In this case, the specific change in direction due to the transition between two mediums is what defines refraction.