diffraction

Meaning: Diffraction is the bending and spreading of a wave as it passes through an opening or around an edge. Each point on a wavefront acts as a source of secondary wavelets, and these interfere. Spreading is large when the opening width a is comparable to or smaller than the wavelength λ, and slight when a is much larger.

Equation: a sinθ = mλ for single-slit dark fringes, where a is slit width (m), θ is angle from the central axis (rad), m is a nonzero integer (dimensionless), and λ is wavelength (m). For a double slit or grating, d sinθ = mλ gives bright fringes, where d is slit spacing (m) and m = 0, ±1, ±2.

On the MCAT: Passages show light through a slit, double slit or grating. You predict that a narrower slit or a longer wavelength widens the pattern, so red light spreads more than blue.

Common confusion: Do not confuse it with refraction, which is bending from a speed change at a boundary (n₁ sinθ₁ = n₂ sinθ₂). Diffraction is spreading at an opening or edge. Also note that single-slit a sinθ = mλ locates dark fringes, while double-slit d sinθ = mλ locates bright ones.