Total internal reflection (TIR) occurs when a light ray travels from a medium with a higher refractive index to a medium with a lower refractive index, and the angle of incidence is greater than the critical angle. When these conditions are met, the light ray does not refract into the second medium. Instead, it is completely reflected back into the first medium. The critical angle (θc) is the angle of incidence for which the angle of refraction is 90 degrees. It can be calculated using Snell's Law: n₁ sin(θc) = n₂ sin(90°) Since sin(90°) = 1, this simplifies to: sin(θc) = n₂ / n₁ TIR is the principle behind fiber optics and is also responsible for phenomena like the sparkle of diamonds and mirages.