Key Concepts: Total Internal Reflection

Total Internal Reflection (TIR)

• Occurs when light travels from a denser to a less dense medium at an angle greater than the critical angle.
• All light is reflected back — none is refracted.

Critical Angle

• sin θ_c = n₂/n₁ (where n₁ > n₂).
• Glass to air: θ_c ≈ 42°. Water to air: θ_c ≈ 49°.
• At exactly θ_c, the refracted ray travels along the boundary (θ₂ = 90°).

Applications

• Optical fibers: Light bounces along the fiber by TIR — used in telecommunications and medical endoscopes.
• Prisms: Binoculars use TIR prisms instead of mirrors for light reflection.
• Diamonds: Very low critical angle (~24°) causes maximum internal reflection → sparkle.