Light only reaches its maximum speed in a vacuum. When traveling through air, it slows down by about 90 kilometers per second.

The speed of light in a vacuum, denoted as 'c', was officially defined by the International Committee for Weights and Measures in 1983 as exactly 299,792,458 meters per second. This speed is a fundamental constant of nature that cannot be exceeded by matter or information. However, this velocity only applies when light is traveling through a perfect vacuum where no particles interfere with its path.When light enters a medium like air, water, or glass, it interacts with the electromagnetic fields of the atoms within that material. This interaction causes a delay in the transmission of the light wave, effectively lowering its phase velocity. The ratio of the speed of light in a vacuum to its speed in a specific medium is known as the refractive index. For example, air has a refractive index of approximately 1.0003 at standard temperature and pressure.This slight difference in air results in a speed reduction of roughly 90 kilometers per second compared to a vacuum. In denser materials like water, which has a refractive index of 1.33, the speed drops to about 225,000 kilometers per second. Diamonds have a much higher refractive index of 2.42, which slows light down to a mere 124,000 kilometers per second. This dramatic slowing is what gives diamonds their characteristic sparkle through internal reflection.The phenomenon of light slowing down is essential for modern technology and biology. Refraction occurs because different parts of a light wave change speed at different times when hitting a medium at an angle. This bending of light is the core principle behind how the human lens focuses images on the retina. It also enables the functionality of fiber optic cables, which power the global internet by guiding light through glass cores.