What characteristic of optical fiber causes total internal reflection?

The characteristic of optical fiber that causes total internal reflection is the difference in refractive indexes between the cladding and the core. In an optical fiber, light is transmitted within the core, which has a higher refractive index than the surrounding cladding. This difference in refractive indexes is crucial because it allows light traveling through the core to be reflected back into the core instead of refracting into the cladding.

Total internal reflection occurs when light hits the boundary between two media at an angle greater than the critical angle, which is determined by the refractive indices of the materials involved. Therefore, having the core with a higher refractive index means that most of the light can be kept within the core, allowing for efficient transmission over long distances with minimal loss.

The other factors mentioned, such as temperature, length, or diameter of the fiber, do not directly affect the principle of total internal reflection itself, although they can influence other properties like signal loss or propagation speed. However, it is the refractive index difference that fundamentally enables the phenomenon of total internal reflection, making it the correct answer.