Practice question

**TIR application** requires n₁>n₂ and θ₁>C. Critical angle formula derived from Snell's law with θ₂=90°, n₁ sinC = n₂ sin90° = n₂, so sinC = n₂/n₁. For glass-air sinC=1/1.52, C≈41°, for water-air 48.6°, determining cutoff for transmission. Focal length: f = -20 cm (concave lens). Image distance: v = -8 cm (virtual image). Lens formula: (1/v) - (1/u) = (1/f) . (1/-8) - (1/u) = (1/-20) ⇒ (1/u) = (1/-8) - (1/-20) = (-5 + 2/40) = (-3/40) . u = -(40/3) ≈ -13.33 cm . Substituting values gives 13.3 cm, which matches expected image position and magnification from mirror/lens formula 1/f = 1/v -