indexes of refraction

Meaning: The index of refraction (n) of a material compares the speed of light in vacuum with its speed in that material. Vacuum has n = 1, water is about 1.33, and glass is about 1.5. A larger n means slower light. On entering a higher-n medium, light slows, bends toward the normal, and its wavelength shortens, while its frequency stays the same.

Equation: n = c/v, where n is the index (unitless), c = 3.00 × 10⁸ m/s is the speed of light in vacuum, and v is the speed of light in the medium (m/s).

On the MCAT: Passages give a table of indexes and ask which way a ray bends or how fast light travels in a material. Snell's law, n₁ sinθ₁ = n₂ sinθ₂, gives the refracted angle, with both angles measured from the normal. Going from higher n to lower n, light bends away from the normal and undergoes total internal reflection beyond the critical angle, sinθc = n₂/n₁. Because n varies slightly with wavelength, violet light bends more than red.

Common confusion: Students often think a higher n means the ray bends away from the normal, or that the light's frequency changes. Higher n means slower light and bending toward the normal. Frequency is set by the source and does not change at the boundary, so speed and wavelength change together.