Total internal reflection will not occur when light travels from

Analyze the question We need the situation where total internal reflection (TIR) is impossible.

Concept TIR happens only when:

1. Light travels from a denser medium to a less dense (rarer) medium, meaning from higher refractive index to lower refractive index.
2. The angle of incidence is greater than the critical angle, where sin C = n₂/n₁ (n₁ > n₂).

If light goes from a rarer to a denser medium, it bends toward the normal and no critical angle exists, so TIR cannot occur.

Refractive indices (approximate)

• Air: 1.00
• Water: 1.33
• Glass: about 1.5

Check each option

• A. Water to air: 1.33 → 1.00 (dense to rare). TIR is possible.
• B. Water into glass: 1.33 → 1.5 (rare to dense). TIR is not possible.
• C. Glass to air: 1.5 → 1.00 (dense to rare). TIR is possible.
• D. Glass into water: 1.5 → 1.33 (dense to rare). TIR is possible. The critical angle is C = sin⁻¹(1.33/1.5) ≈ 62.5°.

Answer Water into glass is the only case where light moves from a less dense to a more dense medium, so total internal reflection cannot occur.

Tip Remember: "TIR only goes from Dense to Rare." If the light is entering a medium with a higher refractive index, rule it out immediately. In multiple-choice questions, compare the refractive indices and look for the option where n increases.