In total internal reflection, the light travels from

Total Internal Reflection (TIR) occurs exclusively when light travels from an optically denser medium to an optically rarer medium [2]. For this phenomenon to take place, the angle of incidence must exceed the critical angle specific to the pair of media [2]. Unlike ordinary reflection or refraction, where some energy is transmitted or absorbed, TIR results in the light being completely reflected back into the original denser medium [1]. This process is characterized by effectively 100% reflection, meaning there is no loss of light intensity at the boundary. This high efficiency makes TIR fundamental for technologies like fiber optics, where light must be transmitted over long distances with minimal attenuation [1]. In contrast, light traveling from a rarer to a denser medium always undergoes refraction and partial reflection, never total internal reflection [2].

Sources

1. [1] https://satheeneet.iitk.ac.in/article/physics/physics-total-internal-reflection/
2. [2] https://www.ncbi.nlm.nih.gov/books/NBK594230/