• Tags: TIR, critical-angle, optical-fibre
• Difficulty: Foundation

TIR occurs when: (1) Light travels from optically denser to rarer medium. (2) Angle of incidence > critical angle θ_c.

Critical angle: sin θ_c = $\frac{n_rarer}{n_denser}$. For glass-air: θ_c ≈ 42°. For diamond-air: θ_c ≈ 24.4° (why diamonds sparkle). For water-air: θ_c ≈ 48.75°.

Applications: Optical fibres (light bounces inside by TIR), mirages (hot air near ground has lower n, TIR of light from sky), totally reflecting prisms (used instead of mirrors for 90° and 180° deviation in periscopes and binoculars).