Which phenomenon occurs when the speed of light changes as it moves from air into glass?

When light travels from one medium to another, such as from air (a less dense medium) into glass (a denser medium), it undergoes a change in speed. This change in speed is accompanied by a change in direction, which is known as refraction. The bending of light as it passes from one medium to another occurs due to the different optical densities of the materials involved.

Refraction is governed by Snell's Law, which establishes the relationship between the angles of incidence and refraction based on the indices of refraction of the two media. In this case, as light enters the glass at an angle, it slows down and bends towards the normal line (the line perpendicular to the surface) due to the higher refractive index of glass compared to air.

The other phenomena mentioned in the choices involve different processes. Reflection refers to light bouncing off a surface, diffraction involves the bending of light around obstacles or through openings, and dispersion is the separation of light into its constituent colors, such as when white light passes through a prism. In this question, the key phenomenon specifically associated with the change in speed and direction of light transitioning from air to glass is indeed refraction.