Snell's law of refraction at the boundary between two media of refractive indices $n_1$ and $n_2$ states

1. Your syllabus: when a ray crosses from medium 1 to medium 2, the angles of incidence and refraction (both measured from the NORMAL) satisfy $n_1\sin\theta_1 = n_2\sin\theta_2$. 2. Consequence: when light enters a denser medium (higher $n$), it bends TOWARD the normal; entering rarer (lower $n$), it bends AWAY. 3. The refractive index $n = c/v$ measures how much slower light travels in the medium. 4. Options B, C, D have wrong trigonometric functions or wrong functional form — Snell's law has no derivation that produces cosines, tangents, or sums. _Source: syllabus Physics Part 2, Ch 9 "Ray Optics", §9.3 (Refraction, Eq. 9.4), p. 2–3._