Light travels slower in fiber optics compared to free space due to the phenomenon of "group velocity" and the refractive index of the fiber material.
In free space, light travels at its maximum speed, known as the speed of light in a vacuum, which is approximately 299,792,458 meters per second (or about 186,282 miles per second). This speed is denoted by the symbol 'c' and is considered a fundamental constant in physics.
When light enters a fiber optic cable, it interacts with the material of the fiber, typically made of glass or plastic. The interaction between light and the fiber material causes the speed of light to decrease. This slowing down of light can be explained by the refractive index of the fiber.
The refractive index is a measure of how much light slows down when it passes through a medium compared to its speed in a vacuum. The refractive index of a fiber optic material is always greater than 1, indicating that light travels slower in the fiber than in a vacuum. The exact value of the refractive index depends on the composition of the fiber material.
As light propagates through the fiber, it undergoes multiple reflections and refractions at the boundary between the fiber core and the cladding. These reflections and refractions occur due to the difference in refractive indices between the core and the cladding. This phenomenon is called total internal reflection and allows the light to propagate through the fiber by bouncing back and forth.
The speed of light in a fiber optic cable is determined by the group velocity, which is the speed at which the overall shape or envelope of the light pulse propagates through the fiber. The group velocity is typically slower than the speed of light in free space due to the interactions with the fiber material.
However, it's important to note that the speed reduction in fiber optics is relatively small. The difference between the speed of light in free space and the speed of light in fiber optics is typically on the order of 30% slower. This slight decrease in speed allows for efficient transmission of light signals through the fiber optic cables, enabling high-speed communication over long distances.