The Conditions for Total Internal Reflection of Light

Total internal reflexion (TIR) is a phenomenon that occurs when light waves travel from a medium with a high refractive index, like water or glass, to a medium with a lower refractive index, like air.
The first condition for TIR to occur is that light should be travelling from a denser medium (higher refractive index) to a less dense medium (lower refractive index). For instance, when light travels from glass to air, from diamond to air, or from water to air.
The second condition is that the angle of incidence (the angle between the incoming light path and the perpendicular to the surface) should exceed the critical angle for the particular medium. Any less, and the light will simply be refracted, or bent, as it exits the medium, rather than being totally internally reflected.
The critical angle is a specific angle of incidence at which light is refracted along the boundary of the medium. Its value can be calculated using the formula: Critical Angle = Sin^-1 (n2/n1), where n1 is the refractive index of the denser medium and n2 is the refractive index of the less dense medium.
If the angle of incidence is larger than the critical angle, no refraction occurs, and instead, the light is completely reflected back into the denser medium. This is TIR.
TIR is a very efficient process - almost 100% of the light is reflected. This makes it valuable in technology, for instance, in fibre-optic cables where light can be transmitted over long distances nearly losslessly.

Remember, both conditions must be met in order for total internal reflexion to occur, and any simplification that omits these factors will miss the essence of this fundamental phenomenon in wave physics. Keep practising different scenario questions to ensure a good understanding of these conditions.