slit's width

Meaning: Slit width, a, is the size of the opening a wave passes through, measured across the slit. When a is comparable to the wavelength or smaller, the wave diffracts. It spreads out and forms a wide central bright band with dimmer bands on each side, separated by dark minima. A narrower slit spreads the wave more, so the central band widens. If a is much larger than λ, there is little spreading and the light travels nearly straight.

Equation: a sin θ = mλ (dark fringes), where a is slit width (m), θ is the angle from the central axis to the minimum (rad), m = 1, 2, 3… is the order of the minimum (dimensionless), and λ is wavelength (m). For small angles, the central maximum on a screen has width about 2λL/a, where L is the slit-to-screen distance (m).

On the MCAT: Passages change a, λ, or L and ask how the pattern changes. Halving a roughly doubles the central width. Red light, with its longer λ, gives a wider pattern than blue.

Common confusion: Students expect a narrower slit to give a narrower pattern, like a shadow. Diffraction does the opposite. Also keep slit width a, which sets single-slit minima, separate from slit separation d, which sets double-slit maxima in d sin θ = mλ.