For total internal reflection to occur, I. light must be travelling from an optically more dense to an…

Analyze the question: We need to decide which conditions must hold for total internal reflection (TIR) to occur.

Concept: TIR happens when light tries to pass from a denser medium to a less dense one and is completely reflected back into the denser medium, with no refraction. Two conditions are needed:

1. The light must travel from an optically denser medium to a less dense one (e.g. glass to air, water to air).
2. The angle of incidence in the denser medium must be greater than the critical angle, c, where sin c = n₂/n₁ (or sin c = 1/n when the other medium is air).

Check each statement:

• Statement I: Light must travel from a denser to a less dense medium. This is correct, because TIR cannot happen the other way round (the ray bends towards the normal, so there is no critical angle).
• Statement II: The angle of incidence must be greater than the critical angle. This is correct. This is the condition for total reflection.
• Statement III: The angle of refraction must be 90°. This is incorrect as a condition for TIR. An angle of refraction of 90° occurs exactly when the angle of incidence equals the critical angle (the refracted ray grazes along the boundary). When TIR actually occurs (incidence > c), there is no refracted ray at all, so no angle of refraction exists.

Conclusion: Only statements I and II are correct.

Answer: D (I and II only)

Exam tip: Remember "Dense to rare, bigger than c, no refraction". Whenever an option says the refracted angle is 90°, that describes the critical angle situation, not TIR itself.