double-slit

Meaning: Light passing through two narrow, closely spaced slits acts as two coherent sources. The waves overlap on a screen and give alternating bright and dark fringes. Bright fringes form where the path difference is a whole number of wavelengths (constructive interference). Dark fringes form where it is a half-integer number of wavelengths (destructive interference). Electrons sent through one at a time still build up the same pattern, which the exam uses to show wave-particle duality.

Equation: d sinθ = mλ for bright fringes, and d sinθ = (m + ½)λ for dark fringes. For small angles, y = mλL/d. Here d is slit separation (m), θ is the angle from the central axis (rad), m is the integer order (unitless), λ is wavelength (m), y is fringe position from the center (m), and L is slit-to-screen distance (m).

On the MCAT: Expect to predict how the pattern changes. A larger λ or L, or a smaller d, spreads the fringes farther apart. Red light spaces fringes wider than violet, so white light gives colored fringes. A medium with a higher refractive index shortens λ and squeezes the fringes together.

Common confusion: Single-slit diffraction uses a sinθ = mλ for dark minima (a is slit width). Double-slit uses d sinθ = mλ for bright maxima (d is slit spacing). Also, moving the slits closer together widens the pattern; it does not narrow it.