total internal reflection

Meaning: Light passing from a higher refractive index to a lower one bends away from the normal. As the angle of incidence grows, the refracted ray swings toward the boundary. At the critical angle the refracted ray travels along the surface (90° from the normal). Beyond that angle no refracted ray can exist, so essentially all the light reflects back into the first medium, following the law of reflection (angle of incidence = angle of reflection).

Equation: sin θc = n₂/n₁, where θc is the critical angle measured from the normal (radians or degrees), n₁ is the refractive index of the incident medium, and n₂ is the refractive index of the second medium (both dimensionless), with n₁ > n₂.

On the MCAT: Passages use fiber optics, prisms, binoculars, or a gem's sparkle. You may calculate θc, for example glass (n ≈ 1.5) to air (n ≈ 1.0) gives sin θc ≈ 0.67, so θc ≈ 42°. You may also predict a trend: a larger n₁ or a smaller n₂ gives a smaller θc, so total internal reflection occurs more easily. Fiber questions often ask why the core must have a higher index than the cladding.

Common confusion: Students think it can happen when light enters a higher-index medium. It cannot, because sin θc would exceed 1 and the refracted ray always exists. Also, measure all angles from the normal, not from the surface. At exactly θc the refracted ray grazes the surface, and total internal reflection applies only for angles larger than θc.