Physics
Speed of Light in a Medium
Quick fact
In water, light travels at about 225,000 km/s—roughly 25% slower than its vacuum speed of 300,000 km/s. In diamond, light slows to just 124,000 km/s, less than half its vacuum speed.
Why this is interesting
You've seen a straw in a glass of water appear bent—that's because light slows down when it enters water. But why does light slow down at all, and how much slower does it get?
Read the full explanation
Understanding Speed of Light in a Medium
Imagine light as a series of wave crests moving through empty space. When it enters a medium like glass or water, the atoms in the material absorb and re-emit the light waves, causing a tiny delay at each atom. The cumulative effect of billions of such interactions makes the overall wave travel slower than in vacuum. This slowing is not a change in the 'true' speed of light (which is constant in vacuum) but rather a macroscopic average. The ratio of light's speed in vacuum to its speed in the medium is called the refractive index. For water, the refractive index is about 1.33, meaning light is 1.33 times slower in water than in vacuum.
A deeper explanation
The fundamental mechanism is rooted in the wave nature of light and its interaction with charged particles in the medium. When an electromagnetic wave passes through a material, the electric field oscillates the electrons in atoms, causing them to emit secondary waves. These secondary waves interfere with the original wave. The net effect is that the phase velocity—the speed at which the wave crests move—is reduced. The reduction depends on the medium's electrical permittivity and magnetic permeability, which are linked to its atomic structure. This explains why different materials have different refractive indices, and why the speed varies with wavelength (dispersion), leading to effects like chromatic aberration and rainbows. Importantly, the energy of the light does not actually slow down; it's the wavefront that propagates more slowly. This concept is crucial for designing lenses, optical fibers, and understanding phenomena like total internal reflection and Cherenkov radiation (when particles exceed the speed of light in a medium).