What phenomenon describes the complete reflection of light at a boundary under specific conditions?

Total internal reflection is the phenomenon that occurs when light travels from a medium with a higher refractive index to a medium with a lower refractive index, resulting in light being completely reflected back into the original medium instead of passing through the boundary. This occurs under specific conditions, namely when the angle of incidence exceeds a certain critical angle, which is determined by the refractive indices of the two media involved.

In total internal reflection, it is essential that light hits the boundary at an angle greater than the critical angle; otherwise, some portion of the light will refract and enter the second medium. This principle is critical in applications such as optical fibers and certain types of prisms, where retaining as much light as possible within the medium is important for efficient signal transmission and manipulation.

The other phenomena mentioned, such as refraction and diffraction, refer to different behaviors of light. Refraction involves the bending of light as it passes through different media, while diffraction is the spreading of light waves when they encounter an obstacle or opening. Ordinary reflection occurs when light bounces off a surface but does not involve the special conditions required for total internal reflection.