index of refraction

Meaning: The index of refraction describes how much a medium slows light. It is 1 for vacuum (very nearly 1 for air), about 1.33 for water, and about 1.5 for common glass. When light crosses a boundary its frequency stays the same, so a drop in speed comes with a proportional drop in wavelength. Entering a higher-n medium, light bends toward the normal; entering a lower-n medium, it bends away from the normal.

Equation: n = c/v, where c = 3.00 × 10⁸ m/s is the speed of light in vacuum, v is the speed in the medium (m/s), and n is dimensionless. Snell's law: n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal.

On the MCAT: Passages give n for several materials and ask for the ray path, the speed or wavelength in a medium, or whether total internal reflection occurs. That requires light going from higher n to lower n at an angle beyond the critical angle, where sin θc = n₂/n₁.

Common confusion: Students think frequency changes in a new medium, or that larger n means faster light. Frequency is set by the source and stays constant; larger n means slower light and a shorter wavelength.