Final answer:
Light travels more slowly in a medium than in a vacuum due to interactions with the medium's atoms and molecules, which absorb and reemit the light, resulting in a longer effective path. The extent of the slowdown is quantified by the medium's index of refraction, and this varying speed causes refraction at boundaries between different media.
Step-by-step explanation:
A common question is why does light travel more slowly in a medium than in a vacuum. Light in a vacuum travels at the universal speed limit, denoted as c. However, when light enters a medium, such as water, glass, or air, it interacts with the atoms and molecules within that medium.
This interaction causes the light waves to be absorbed and reemitted multiple times at microscopic scales, effectively causing the waves to take a longer path and therefore slow down compared to their speed in a vacuum. The slowing down is only apparent; it's due to the microscopic detours taken in the medium. The degree of slowing down depends on the material's index of refraction, which results from the strength of the interaction between light and the medium's particles.
When light moves from one medium to another, its speed changes depending on the optical density of the media involved. This change in speed results in the bending of the path of the light, a phenomenon known as refraction. The more optically dense the medium (e.g., diamond versus water), the more dramatic the reduction in speed, and the greater the angle of refraction.