How does total internal reflection occur inside glass or plastic?

Total internal reflection occurs when light travels from a medium with a higher refractive index, such as glass or plastic, toward a medium with a lower refractive index, such as air.12

As light rays approach the boundary between these two media, they are refracted away from the normal line. As the angle of incidence increases, the angle of refraction also increases until it reaches 90°, meaning the light skims along the surface of the material. This specific incident angle is known as the critical angle.32451

When the angle of incidence exceeds this critical angle, the light can no longer pass into the second medium and is instead completely reflected back into the original material. This principle allows light to be "piped" or conducted through glass or plastic rods and fibre optics by undergoing thousands of successive internal reflections, enabling the transmission of light and images over long, curved paths with minimal loss of intensity.142