For light going from a medium of refractive index n to air, the critical angle i_c is given by:

At the critical angle, r = 90°. Snell's law: **n × sin i_c = 1 × sin 90° = 1 → sin i_c = 1/n**. For water (n = 1.33): i_c ≈ 48.75°. For diamond (n = 2.42): i_c ≈ 24.4° (small i_c → strong TIR → diamond sparkle).