Interference Optical Path
Intensity Distribution I(θ) = I₀·cos²(πd·sinθ/λ)
Simulated Interference Pattern on Screen
Interference Parameters
Slit Properties
Light Properties
Display Options
Quick Presets
Interference Formulas
What is Young's Double Slit Interference?
Young's double slit interference is a classic wave optics experiment demonstrating the wave nature of light. When coherent light passes through two parallel narrow slits, the light waves from each slit interfere, creating a pattern of alternating bright and dark fringes on a screen. This experiment, performed by Thomas Young in 1801, provided crucial evidence for the wave theory of light.
Interference Mechanism
When a plane wave encounters two slits separated by distance d, each slit acts as a source of coherent secondary spherical wavelets (Huygens' principle). These wavelets overlap and interfere. The path difference between waves from the two slits is Δ = d·sinθ, where θ is the angle from the central axis. Constructive interference (bright fringes) occurs when Δ = mλ, where m = 0, ±1, ±2, ... is the order number. Destructive interference (dark fringes) occurs when Δ = (m+½)λ. The central bright fringe (m=0) is the brightest.
Intensity Pattern
The intensity distribution follows I(θ) = I₀·cos²(πd·sinθ/λ), a cos² function resulting from the superposition of two waves of equal amplitude. At the center (θ = 0), the path difference is zero, giving maximum intensity I₀. The fringes are equally spaced in angle, with the angular separation between adjacent bright fringes being Δθ ≈ λ/d (for small angles). On the screen, the fringe spacing is Δx = λL/d, directly proportional to wavelength λ and screen distance L, and inversely proportional to slit separation d.
Effect of Slit Separation
The slit separation d inversely affects the fringe spacing: closer slits (smaller d) produce wider fringe patterns (Δx ∝ 1/d), while wider slit separations produce narrower, more closely spaced fringes. When d ≪ λ, the pattern becomes very wide with few visible fringes. When d ≫ λ, the fringes become very closely spaced and may become difficult to distinguish. This inverse relationship is a key characteristic of double-slit interference and allows precise measurements of small distances.
Effect of Wavelength
Longer wavelengths (red light) produce wider fringe spacing than shorter wavelengths (blue light), since Δx ∝ λ. In white light, each wavelength creates its own interference pattern, resulting in colored fringes with white at the center and colors spreading outward. Red light diffracts more on the outer edges, while blue light forms fringes closer to the center. This wavelength dependence allows the double slit to act as a simple spectrometer, separating white light into its component colors.
Applications
Young's double slit experiment has numerous applications: measuring the wavelength of light sources by analyzing fringe spacing, determining the separation between closely spaced objects, studying coherence properties of light sources, understanding the wave-particle duality of quantum mechanics (electron double slit experiment), optical testing and metrology, and as a fundamental demonstration in physics education. The experiment forms the basis for more complex interferometric devices like the Michelson interferometer used in gravitational wave detectors (LIGO) and precision measurements.