Total internal reflection takes place, when light travels fromA. Water to glassB. Glass to diamondC. Water to airD. Air to mercury

Problem Statement:
We need to determine the condition under which total internal reflection (TIR) occurs and identify the correct pair of materials where this phenomenon is observed.
Key Concept: Total Internal Reflection (TIR)
1. Definition: TIR occurs when a light ray traveling from a denser medium to a rarer medium is incident at an angle greater than the critical angle (CC). At this angle:
o The light is completely reflected back into the denser medium.
2. Conditions for TIR:
o The light must travel from a denser medium to a rarer medium (e.g., water to air, glass to air).
o The angle of incidence must be greater than the critical angle (CC), which is defined as: sin⁡C=n2n1\sin C = \frac{n_2}{n_1} Where:
 n1n_1 is the refractive index of the denser medium,
 n2n_2 is the refractive index of the rarer medium.
Step-by-Step Analysis of Options:
1. Option A: Water to glass:
o Water has a lower refractive index (n≈1.33n \approx 1.33) than glass (n≈1.5n \approx 1.5).
o Since light is traveling from a rarer medium (water) to a denser medium (glass), TIR does not occur.
2. Option B: Glass to diamond:
o Glass (n≈1.5n \approx 1.5) has a lower refractive index than diamond (n≈2.4n \approx 2.4).
o Since light is traveling from a rarer medium (glass) to a denser medium (diamond), TIR does not occur.
3. Option C: Water to air:
o Water (n≈1.33n \approx 1.33) has a higher refractive index than air (n≈1.0n \approx 1.0).
o When light travels from water to air and the angle of incidence exceeds the critical angle, TIR occurs.
o Example: A swimming pool's surface appears reflective when viewed underwater at a steep angle.
4. Option D: Air to mercury:
o Air (n≈1.0n \approx 1.0) has a lower refractive index than mercury (n≈1.5n \approx 1.5).
o Since light is traveling from a rarer medium (air) to a denser medium (mercury), TIR does not occur.
Total internal reflection takes place when light travels from water to air.
Correct option: (C) Water to air.