Which phenomenon occurs when light is totally reflected within a substance at an angle less than the critical angle?

The phenomenon that occurs when light is totally reflected within a substance at an angle less than the critical angle is known as total internal reflection. This process requires that the light travels from a medium of higher refractive index to one of lower refractive index and is incident at an angle exceeding the critical angle.

In this context, if the angle of incidence is less than the critical angle, total internal reflection does not occur; rather, some light will pass through the boundary into the second medium, refracting instead. Therefore, the correct understanding of total internal reflection is essential to grasping how light behaves at boundaries between different media.

Total internal reflection is largely used in applications like optical fibers, where light remains within the fiber and transmits effectively over long distances. This property is tied directly to the refractive indices of the materials involved, emphasizing the importance of the critical angle in understanding how light behaves at these interfaces.