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. The virtual focal point of a concave lens is where diverging rays appear to originate when traced backward. This position is determined by the lens’s focal length, which depends on its curvature and refractive index, defining the extent of divergence. Substituting values gives Focal length, which matches expected image position and magnification from mirror/lens formula 1/f = 1/v - 1/u (lens) or 1/f = 1/v + 1/u (mirror),