Under what conditions does Total Internal Reflection (TIR) occur at the boundary between two optical media?

Answer and explanation

Correct answer: C. Light travels from denser to rarer medium and incidence angle exceeds critical angle

  1. Step 1: According to Snell's law, . When light travels from a denser medium () to a rarer medium (), the refracted ray bends away from the normal ().
  2. Step 2: As angle of incidence increases, angle of refraction approaches . The angle of incidence for which is called the critical angle , where .
  3. Step 3: If , no refracted ray can enter the rarer medium (as is mathematically and physically impossible), and 100% of the incident light is reflected back into the denser medium.
  4. Step 4: Therefore, the two essential conditions are: (1) light must propagate from denser to rarer medium, and (2) the angle of incidence must exceed the critical angle ().
Answer
Light travels from an optically denser medium toward an optically rarer medium, and the angle of incidence exceeds the critical angle (i > C)
Why not the others: 'Light travels from rarer toward denser' results in bending toward the normal, where angle of refraction is always smaller than angle of incidence, making TIR impossible; 'Normal incidence' passes straight through without any deviation (); 'Identical refractive indices' allows light to travel undeflected without reflection.

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