Which phenomenon is described by light being completely reflected back into the medium?

Total internal reflection occurs when light travels from a denser medium to a less dense medium at an angle greater than the critical angle. In this situation, instead of refracting (bending) into the less dense medium, the light is completely reflected back into the denser medium. This phenomenon is crucial in applications such as fiber optics, where it allows light to be guided along a path with minimal loss.

When the conditions for total internal reflection are met, the light does not lose any energy to the external medium, making it an efficient way to transmit light. This differs from regular reflection, where light bounces off a surface without the specific dependency on the media involved. Refraction involves the bending of light as it passes from one medium to another, and dispersion refers to the separation of light into its constituent colors, often due to different wavelengths being refracted by different amounts. Total internal reflection is unique in that it fundamentally relies on the interaction of light with boundaries between different media and the critical angle, resulting in the total retention of light within the original medium.