young's double-slit experiment

Meaning: Coherent, monochromatic light passes through two narrow slits separated by a distance d. The two emerging waves overlap on a distant screen. Where their path difference is a whole number of wavelengths, they arrive in phase and interfere constructively, giving a bright fringe. Where it is a half-integer number of wavelengths, they interfere destructively, giving a dark fringe. The pattern is classic evidence for the wave nature of light.

Equation: Bright fringes: d sinθ = mλ. Dark fringes: d sinθ = (m + ½)λ, with m = 0, 1, 2… For small angles, Δy = λL/d. Here d is the slit separation (m), θ is the angle from the central axis (rad), λ is the wavelength (m), L is the slit-to-screen distance (m), and Δy is the spacing between adjacent bright fringes (m).

On the MCAT: Expect fringe-spacing predictions. A longer λ or larger L widens the spacing, and a larger d narrows it. Putting the setup in a medium with a higher index of refraction shortens λ, so fringes move closer together. White light gives a white central fringe with colored fringes beside it. Electron or single-photon versions illustrate wave-particle duality.

Common confusion: Single-slit diffraction uses a sinθ = mλ for dark fringes, where a is slit width. Double-slit uses d sinθ = mλ for bright fringes. Check the number of slits and whether the question asks for bright or dark.