What is the effect that causes light to change direction when it passes into a different medium?

When light travels from one medium to another, it changes its speed depending on the optical density of the mediums involved. This change in speed causes the light to change direction, a phenomenon known as refraction. For example, when light enters water from air, it slows down, which results in it bending towards the normal (an imaginary line perpendicular to the boundary).

Refraction is responsible for various visual effects, such as the bending of a straw when it's placed in a glass of water or the way objects appear distorted when viewed through a lens. This effect is quantitatively described by Snell's Law, which relates the angles of incidence and refraction to the indices of refraction of the two media.

In contrast, other options like reflection involve light bouncing off a surface, diffraction relates to the bending of light around obstacles or the spreading of light waves through small openings, and dispersion refers to the separation of light into its constituent colors, such as when light passes through a prism. Each of these phenomena describes different interactions of light but does not directly address the change in direction due to a change in medium.