Latency is a term that is used to describe a time delay in a transmission medium such as a vacuum, air, or a fiber optic waveguide. In free space, light travels at 299,792,458 meters per second. This equates to 299.792 meters per microsecond (µs) or 3.34µs per kilometer. In fiber optics, the latency of the fiber is the time it takes for light to travel a specified distance through the glass core of the fiber. Light moving through the fiber optic core will travel slower than light through a vacuum because of the differences of the refractive index of light in free space and in the glass.