Snell's law of refraction

Meaning: When light crosses into another transparent medium, its speed changes (v = c/n) but its frequency does not, so its wavelength changes too. The change in speed makes the ray change direction at the boundary. Into a slower medium (higher n) the ray bends toward the normal. Into a faster medium (lower n) it bends away from the normal. A ray travelling along the normal is not deflected.

Equation: n₁ sin θ₁ = n₂ sin θ₂, where n₁ and n₂ are the indices of refraction of the first and second media (dimensionless, n = c/v) and θ₁ and θ₂ are the angles of the incident and refracted rays from the normal (radians in SI; degrees are also used).

On the MCAT: Expect a passage giving indices, such as air to water or glass, and a question asking for the refracted angle or which way the ray bends. Estimates often use sin 30° = 0.5. The law also leads to the critical angle, sin θc = n₂/n₁ (only when n₁ > n₂), and to total internal reflection in fiber optics. Because n varies slightly with wavelength, prisms spread white light into colors.

Common confusion: Students measure the angle from the surface instead of from the normal, which gives the wrong sine. Another error is thinking that bending toward the normal means light speeds up. It means the light slows down, because n is larger in the second medium.