What causes light to change direction at the boundary between two mediums?

Light changes direction at the boundary between two media primarily due to the density difference between the mediums. This phenomenon is known as refraction. When light travels from one medium into another, such as from air into water, it slows down or speeds up depending on the optical density of the mediums involved.

In a denser medium, light travels more slowly compared to a less dense medium. This change in speed as light enters the new medium causes it to change direction. The degree of bending depends on the angle at which the light enters the new medium and the refractive indices of both mediums. This relationship is described by Snell's Law, which quantitatively relates the angles and the indices of refraction of the two media.

Other factors mentioned in the choices do not directly cause the change in direction of light. The change in color of light pertains to how light interacts with different materials but does not influence the bending that occurs due to density differences. Temperature differences can affect the density of a medium, but they are not a direct cause of light changing direction at the boundary. While the angle of incidence does play a role in the degree of bending, it is the density difference that fundamentally dictates the change in speed and direction of the light.