What phenomenon occurs when the angle of light is low enough for it to reflect entirely back into the core of a fiber?

Total Internal Reflection occurs when light travels through a medium, such as a fiber optic cable, and reaches a boundary with a less dense medium at a sufficiently shallow angle. If the angle of incidence is greater than the critical angle specific to the materials involved, the light does not pass through the boundary but instead reflects entirely back into the denser medium, or the core of the fiber. This phenomenon is fundamental to the operation of fiber optics, as it allows the light signals to travel long distances without significant loss of intensity, making fiber optic technology highly effective for data transmission.

In contrast, refraction refers to the bending of light as it passes from one medium to another, which is not the case here since we are discussing a situation where light does not exit the fiber. Reflection, in general terms, describes the bouncing back of light from a surface, but does not capture the specificity of the conditions necessary for total internal reflection to occur. Diffusion relates to the spreading out of light as it interacts with particles or surfaces, which is not relevant to the mechanism by which light is guided within fiber optics.