light refraction

Meaning: Refraction is the change in direction of a light ray when it crosses a boundary between media in which light travels at different speeds. Speed in a medium is v = c/n, where n is the index of refraction. Frequency is set by the source and stays the same, so wavelength changes with speed (λ = v/f). Entering a higher n, the ray slows and bends toward the normal; entering a lower n, it speeds up and bends away. A ray along the normal changes speed but does not bend.

Equation: n₁ sin θ₁ = n₂ sin θ₂, with n = c/v. Here n is the index of refraction (unitless), θ is the angle between the ray and the normal (radians or degrees), c = 3.00 × 10⁸ m/s, and v is the speed of light in the medium (m/s).

On the MCAT: Expect questions on which way a ray bends at an air–water or air–glass boundary, finding an unknown n from measured angles, or comparing speed and wavelength in two media. Total internal reflection uses the same law when light passes from higher n to lower n at large enough incidence angles.

Common confusion: Students measure angles from the surface instead of from the normal, and some think frequency changes in the new medium. The law uses the normal, and only speed and wavelength change.