Under what condition does total internal reflection occur?

Total internal reflection occurs when a wave, particularly light, travels from a medium with a higher refractive index (denser) to a medium with a lower refractive index (less dense) and strikes the boundary at an angle greater than the critical angle. At this angle, the wave cannot transmit into the less dense medium and instead reflects completely back into the denser medium. This phenomenon is crucial in applications such as optical fibers and prisms, where it facilitates the efficient transmission of light.

The critical angle is the specific angle of incidence at which the refracted light would emerge along the boundary between the two media. If the angle of incidence exceeds this critical angle, total internal reflection takes place, preserving the energy of the wave within the denser medium.

In contrast, when the wave passes from a denser medium to a less dense medium at any angle, it may partially reflect and partially refract without achieving total internal reflection. Similarly, hitting a boundary at the critical angle will result in some light refracting along the boundary rather than being entirely reflected. Lastly, waves traveling through a vacuum do not involve a boundary between two different media, so the concept of total internal reflection does not apply in that context.