Practice question

**Refractive index** n = c/v, water 1.33 means light 1.33 times slower than vacuum. Passing from water to air at 49°, n₁ sinθ₁ =1.33×sin49°≈1.33×0.755=1.004>1, so sinθ₂>1 impossible, total internal reflection occurs, no refraction. Focal length: f = 18 cm (convex mirror). Image distance: v = 6 cm (virtual image). Mirror equation: (1/v) + (1/u) = (1/f) . (1/6) + (1/u) = (1/18) ⇒ (1/u) = (1/18) - (1/6) = (1 - 3/18) = (-2/18) = (-1/9) . u = -9 cm . Substituting values gives 12 cm, which matches expected image position and magnification from mirror/lens formula 1/f = 1/v - 1/u (lens) or 1/f