Light is the fastest thing in the universe, but only when it is traveling through a vacuum. When a light wave passes through a material like water or glass, it bumps into atoms and slows down. This change in speed is the secret behind how we can steer light inside a fiber optic cable.

To measure how much a material slows light down, we use a number called the refractive index, represented by the letter $n$. It is calculated by dividing the speed of light in a vacuum ($c$) by its speed in the material ($v$):

$n = \frac{c}{v}$

For example, water has a refractive index of about 1.33, meaning light travels 1.33 times slower in water than in a vacuum. When light transitions between materials with different refractive indices, this change in velocity forces the light path to bend, laying the physical foundation for trapping light in a guide.