Total internal reflection occurs when light travels from:
- Rarer to denser medium
- Denser to rarer medium
- Vacuum to glass
- Air to water
Answer: Denser to rarer medium
When light moves from denser (e.g., glass) to rarer medium (e.g., air) at angle > critical angle, it reflects entirely back. Used in optical fibers, prisms. Fundamental optics principle. [[NCERT Physics Class 12]]