Practice question

**Critical angle** C satisfies sinC = n₂/n₁, n₁>n₂, n₂=1 for air, n₁=1.52 for glass gives sinC=1/1.52=0.6579, C≈41.1°, for water n=1.33 C=48.75°. Beyond C, total internal reflection occurs, all light reflected, no refracted ray, used in optical fibers and prisms. Focal length: f = 10 cm . Image distance: v = 20 cm (real image). Lens formula: (1/v) - (1/u) = (1/f) . (1/20) - (1/u) = (1/10) ⇒ (1/u) = (1/20) - (1/10) = (1 - 2/20) = (-1/20) . u = -20 cm . Substituting values gives 20 cm, which matches expected image position and magnification from mirror/lens formula 1/f = 1/v - 1/u