refraction

Meaning: Refraction is the change in direction of a wave when it crosses a boundary into a medium where its speed is different. The source sets the frequency, so it stays constant; speed and wavelength change together (v = fλ). Light entering a medium with higher n slows and bends toward the normal; entering lower n it speeds up and bends away.

Equation: n₁ sinθ₁ = n₂ sinθ₂, where n = c/v is the index of refraction (unitless), c = 3.00 × 10⁸ m/s, v is the wave speed in the medium (m/s), and θ is the angle of the ray from the normal (degrees or radians).

On the MCAT: Questions give two indices and ask which way a ray bends or what the angle is. Related: total internal reflection (higher n to lower n past the critical angle, sinθc = n₂/n₁), lenses, and dispersion, where violet bends more than red in glass.

Common confusion: Angles are measured from the normal, not the surface. Frequency does not change; wavelength does. A ray along the normal is not bent, though its speed still changes.