Snell's law

Meaning: When light crosses a boundary between two transparent media, its speed changes, so its direction usually changes too. The refractive index is n = c/v, so a higher n means slower light. Entering a higher-n medium bends the ray toward the normal. Entering a lower-n medium bends it away from the normal. A ray traveling along the normal (θ = 0) is not bent. Frequency stays the same across the boundary, while speed and wavelength change.

Equation: n₁ sin θ₁ = n₂ sin θ₂, where n₁ and n₂ are the refractive indices of the first and second media (dimensionless), and θ₁ and θ₂ are the angles of the incident and refracted rays measured from the normal (radians, or degrees). Related: n = c/v, where c = 3.00 × 10⁸ m/s and v is the speed of light in the medium (m/s).

On the MCAT: Expect a passage giving two indices and one angle, asking for the other angle or the direction of bending. The law also leads to the critical angle, sin θc = n₂/n₁ (with n₁ > n₂), and to total internal reflection. Prisms and dispersion are common: n varies slightly with wavelength, so different colors bend by different amounts.

Common confusion: Students often measure angles from the surface instead of from the normal. Snell's law uses the normal. Another error is thinking that frequency changes in the new medium. It does not, because the wave's source and boundary fix it; wavelength changes instead.