Which of the following best describes total internal reflection?

Total internal reflection occurs when light traveling within a denser medium approaches a boundary with a less dense medium at an angle greater than the critical angle. At this point, instead of refracting out of the medium, the light is entirely reflected back into the denser medium. This phenomenon is significant for various applications, such as fiber optics, where it allows for efficient light transmission along fibers without loss.

The other options do not accurately reflect the nature of total internal reflection. Light exiting the material and refracting describes typical refraction when the angle of incidence is less than the critical angle, which is not total internal reflection. Similarly, light traveling straight through the boundary indicates that it is either refracting or not interacting with the boundary at all. Lastly, light being absorbed by the medium does not apply to total internal reflection since, in that case, the light is preserved within the medium rather than lost. Therefore, the best description is that light is completely reflected within a medium.