dispersion

Meaning: Light of different wavelengths travels at different speeds in a material, so each color has its own index of refraction. In most transparent materials across the visible range, n is larger for shorter wavelengths. Violet is slowed more and bends more toward the normal than red. A prism refracts light at two surfaces and fans white light into a spectrum, and raindrops do the same in a rainbow. Frequency stays fixed when light enters a medium. Speed and wavelength change.

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

On the MCAT: A passage shows a prism, a rainbow, or a graph of n versus wavelength. You then rank which color bends most, travels slowest in the glass, or deviates most from its original path. Chromatic aberration is the same effect in a lens: different colors focus at different points.

Common confusion: Students assume red bends most, or that color changes when light enters glass. Color is set by frequency, which does not change. In normal dispersion violet has the largest n and bends most. In a vacuum there is no dispersion, because every wavelength travels at c. In statistics, dispersion means the spread of data, which is a different sense.